Break-In Oils and Assembly Lube Needs with Engines (2024)

Engine building is a process that should go well when everything is done correctly (all the parts have been machined to the correct tolerances, assembled with the correct clearances and protected with the right kind of lubricant).

Or, it can end disastrously if parts don’t fit right (too tight or too loose), if there’s not enough lubrication when the engine is first started, or if the motor oil that’s used during the break-in process fails to protect the cam and lifters or doesn’t allow the rings to seat.

A lot of things can go wrong during those first few minutes following the initial start-up of a freshly built engine. The engine has to build oil pressure quickly so all of the critical wear surfaces will receive lubrication.

Assembly lube is designed to cling to surfaces better than ordinary motor oil so there will be a protective film of lubricant until the engine is cranked over and started.

From that point on, oil pressure should take over and flush away most of the assembly lube. That’s why priming the oil system prior to starting it is so important. Priming prevents a dry start and reduces the lag time for oil to reach the bearings, cam and upper valvetrain components.

Ordinary motor oil can be used to lightly lubricate cylinder walls, lifter bores, wrist pins, piston rings, timing chains and bearing surfaces.

But if the engine sits for more than a few days, much of the protective oil film will trickle back into the crankcase.

Adding a viscosity improver such as STP to ordinary SAE 30 motor oil will help it cling to critical surfaces longer, and adding a dose of extreme pressure additive such as zinc dialkyl dithiophosphate (ZDDP) or zinc-moly will provide additional protection for the cam lobes and lifters if the engine has a flat tappet cam.

But the best choice for lubricating and protecting critical parts is an engine assembly lube. Assembly lubes are typically a light moly-based, high-pressure grease or specially formulated oil with extreme pressure additives and rust inhibitors.

Some lubes have a paste-like consistency and are applied with a brush while others are more like honey and can be applied from a squirt bottle.

There are dozens of assembly lubes available, including those from various oil suppliers as well as camshaft and bearing manufacturers. The ingredients in these products are proprietary and there are differences from one product to another. Some grease-based products are not soluble in oil and will end up in the oil filter after the has been running. Others are oil-soluble and will continue to circulate with the motor oil until the oil and filter are changed.

Regardless of which product you choose, a thin coating of assembly lube should be applied on all high-friction, high-load surfaces such as cam lobes, lifter bottoms, pushrod ends, rocker arm and valve stem tips, as well as all the rod, main and cam bearings.

The assembly lube will stay on the surfaces of these parts and provide the much-needed lubrication until oil pressure can take over. Motor oil (straight weight or a multi-viscosity oil) can be used to lubricate less critical surfaces such as the cylinder bores.

Break-In Oil

The assembly lube and motor oil that coat the engine’s internals should protect the metal surfaces against corrosion until it’s time to fire up the engine for the first time. At that point, you (or your customer) will have to decide on what type of break-in oil to use.

As with assembly lubes, there are a variety of different break-in oils that can be used for the initial start up and break-in process. Break-in oils are usually specially formulated straight SAE 30 or SAE 40 oils but some are multi-viscosity oils (5W-30, 15W-40 & 10W-40).

Many people say a conventional mineral-based SAE 30 motor oil that contains extra extreme pressure additive (ZDDP and/or moly) and little or no friction modifiers is best for engine break-in.

Others say they use a conventional SAE 30 oil or a multi-viscosity oil (5W-30 or 15W-40) with a bottle of engine break-in additive which contains the extra extreme pressure additive to protect the cam and lifters).

All agree that a high level of ZDDP is absolutely essential if the engine has a flat tappet cam (especially a high lift performance cam with higher than normal loads on the cam lobes). Extra extreme pressure additive is also beneficial for roller cams with stiff valve springs, too.

Some people prefer to use a SAE 30 non-detergent motor oil to break in a newly built engine. Others use a conventional 15W-40 diesel oil because it contains a higher level of ZDDP than motor oils for gasoline engines. And some just use the same motor oil they plan to use in the engine for regular use for the initial break-in, then change it after a few hours or 50 to 100 miles to get rid of the contaminants.

Straight Weight Or Multi-Viscosity?

Opinions vary as to whether or not you should use a straight weight SAE 30 motor oil or a multi-viscosity oil breaking in a new engine.

Straight weight oils contain no friction modifiers which are used to give multi-viscosity oils their wide temperature range. Friction modifiers are long chain polymers similar to synthetic rubber that are ground up and blended into multi-viscosity oils. This allows the use of a thinner viscosity base oil for easier cold starting and reduced friction. As the oil heats up, the friction modifier thickens and allows the oil to behave more like a heavier viscosity oil at operating temperature.

That’s great for everyday driving, improving fuel economy and increasing horsepower, but for engine break-in some say the less friction modifier in the oil, the better. Others counter and say that’s nonsense and that friction modifiers have little or no impact on engine break-in.

Detergents and dispersants are other additives that are put in motor oils to help keep the engine clean. A newly built engine should have no varnish or sludge deposits, but it may contain some residual debris from machined parts that were not thoroughly cleaned before they were installed, or even casting residue from a new block or heads.

There will also be wear particles generated by the piston rings, cylinder walls and other moving/sliding parts as the engine breaks in. Any such particles or contaminants that are inside the engine must be suspended and carried away by the oil to the oil filter.

Those who favor a low-detergent or no-detergent oil say leaving the particles in the engine longer will speed up the ring seating process. Those who favor an oil with high detergency say suspending and removing any wear particles as quickly as possible protects the bearings, wrist pins and other parts with close tolerances and high loads against damage and premature wear.

Mineral-Based Or Synthetic?

Then there’s the issue of whether or not a synthetic oil can be used for engine break-in.

Most people prefer using a conventional oil for the initial break-in, and then use a conventional oil for the next 1500 to 5000 miles before switching to a synthetic if they want to use a synthetic oil.

Synthetic oils are made from higher quality base stocks that provide superior lubricity as well as longevity.

Some say synthetic oils are “too slippery” for use as a break-in oil while others say using a high quality synthetic oil for the initial break-in is a waste of money because the oil will be drained within a few hours or few hundred miles once the initial break-in process is finished.

There’s no straight answer to this question because of how long it takes for the rings to seat depends on how they cylinder walls were finished.

A traditional one or two-step cylinder honing process leaves a fairly rough surface finish with peaks and valleys. This type of surface finish will require more time for the piston rings to gradually scrape off the peaks and wear down the surface.

It may take a few hundred to several thousand miles before the rings are fully seated. During this time the engine should be run at different speeds rather than a constant RPM to help the seating process. Alternately accelerating the engine followed by a long slow deceleration and high intake vacuum will likewise help the rings seat more quickly.

On the other hand, if the cylinders are “plateau” honed with a multi-step honing process that includes brushing as the final step, the surface finish on the cylinders will be much closer to a broken-in profile.

There will still be plenty of valleys in the crosshatch to retain oil for ring lubrication, but the sharp peaks will have been mostly removed reducing the time and wear required to seat the rings. This, in turn, means the rings will finish seating very quickly reducing wear particles generated by the break-in process.

Consequently, it should make no difference if a conventional oil or synthetic oil is used for the initial break-in. The deciding factor would be the cost of the oil itself.

Many late model engines are factory-filled with multi-viscosity 5W-20, 5W-30 or even 0W-40 synthetic or synthetic-blend oil from the start, and may not specify the first oil change for several thousand miles.

This approach to engine break-in may work well enough for stock engines (which are plateau honed) and everyday driving, but would be too risky for most performance applications. Using a break-in oil for the initial start up, cam and ring seating process and engine tuning adjustments, then draining the oil and replacing it with either a conventional or synthetic oil is much the preferred approach.

How Long?

The companies who supply specially formulated engine break-in oils have different recommendations as to how long their break-in oils can be left in an engine.

The typical recommendation is to use the break-in oil for the first hour or so of run time and tuning, then drain it and change the filter. Some say their break-in oils can be left in the engine while doing dyno pulls or even for one night of racing.

One supplier says their product can be left in for up to 400 miles of street driving while another says the use of their break-in oil should not exceed 1000 miles. Follow the break-in oil supplier’s recommendations for how long their product should be left in the engine and when it should be changed.

The point here is that break-in oil is for break-in only.

A break-in oil should NOT remain in the engine longer than necessary to complete its initial task.

It gets dirty quickly so the sooner it is drained and the filter is replaced, the better.

Draining the oil and changing the filter removes harmful wear particles and residual assembly contaminants before they can cause damage. Following up with a second oil change at low miles or after a limited number of hours of operation is also a common practice to make sure most of the contaminants are kept out of the engine.

Some people will complete the initial break-in process with break-in oil, drain it and use conventional mineral-based oil for the next 50 to 500 miles.

They will change the oil again and extend the next change interval to 1000 to 3000 miles before they change it again and possibly switch to a synthetic motor oil.

Others will do the initial break-in and drain, then go straight to a synthetic oil if the rings appear to be fully seated.

For performance applications, some type of “racing” oil is recommended for optimum protection after the engine break-in process has been completed. Ordinary motor oils (both conventional and synthetic) that meet current API “SN” and ILSAC GF-5 specifications are formulated for late model engines with roller cams and emission controls.

They are designed to provide improved fuel economy and to extend the life of the catalytic converter. The level of ZDDP has been reduced to less than 800 ppm (it used to be 1200 ppm or higher), so the ability to prevent wear on a high lift flat tappet cam with higher than stock valve spring pressure is minimal.

Racing oils as well as street performance oils that are formulated for older engines with flat tappet cams contain the extra ZDDP and/or moly to protect the cam and prevent premature cam failures. Racing oils also use high quality base stocks and additives that are designed to handle higher temperatures and loads.

Some racing oils also contain special additive packages for use with alcohol fuels.

Lower viscosity synthetic oils such as 0W-40, 5W-20, 5W-30 and 5W-40 are thinner oils that allow easier cold starting and reduced friction for better performance and fuel economy.

However, thinner oils also require somewhat closer bearing clearances to maintain the oil film between the moving parts.

A heavier viscosity oil is better for looser bearing clearances and will maintain higher oil pressure readings at peak temperature and engine speeds.

Another note about using low viscosity motor oils in a performance engine is that windage drag, oil aeration and a drop in oil pressure can become problematic at higher engine speeds unless the engine is fitted with a well-designed oil scraper, windage tray and oil pan, or even a dry sump oil system.

Engine Break-In Recommendations

First and foremost is cleaning everything thoroughly BEFORE it goes into an engine. That includes brand new parts out of the box as well as machined and reconditioned parts.

Cylinder bores should be scrubbed with hot soapy water and a brush to remove honing residue. A quick wash and rinse or wiping with solvent can leave a lot of debris on the surface that will end up in the oil. Oil holes in the crankshaft and engine block should be brushed and flushed to remove any trapped debris.

Use assembly lube on all critical parts, and oil on everything else when the engine goes together. The engine should also be assembled in a clean room where there is no dust or airborne contaminants. Assembly tools also need to be clean.

The engine’s oil system should be primed before the engine is cranked and started to prevent a dry start.

As soon as the engine starts, it needs to be revved to 1500 to 2200 RPM and keep there for 20 to 30 minutes while the cam and rings are seating. Varying the RPM helps seat the rings more quickly. Keep an eye on oil pressure during this critical phase and listen for any unusual noises such as rapping, knocking, tapping or clattering that might indicate something is loose, misadjusted or is not receiving adequate lubrication.

Once the initial fuel, ignition and valvetrain adjustments have been made and initial dyno pulls completed, shut the engine off, drain the break-in oil, and remove and inspect the oil filter for any signs of trouble (such as metallic debris that indicates unusual wear is occurring).

Refill with oil, install a new filter and continue with the tuning or break-in process as needed. Change oil and filter again after 50 to 500 miles (or so many hours of run time).