A TV show said we should use break-in oil

November 1, 2015 | By Staff


I rebuilt a small block Chevy 350 engine for use in my project car, which is a 1970 Chevrolet Nova. After installing the engine I filled it with synthetic oil. While watching a restoration show on TV, they made the comment that a fresh rebuild should never be fired up with a full synthetic oil, but a “break-in” oil should be used.

A mechanic friend of mine told me the only difference between the synthetic oil and mineral oil is that the synthetic oil won’t change its viscosity much in cold temperatures. He felt I would be fine using the synthetic oil during the breakin period. What is your take regarding break-in oil?


To begin with, there is no such thing as break-in oil. In the past, mechanics often ran very light (10wt) oil for start-up to make sure the oil got to the bearings quickly, and to help the rings to seat quickly, but these days the professional mechanics I know just use regular 10w30 detergent oil for the initial start-up. Perhaps there is a need for some kind of special break-in oil for highly stressed exotic racing engines, but I would doubt even that. Also there are a lot of over-the-counter magic unguents available for use when breaking in engines, but I have never found them necessary or beneficial.

Engine break-in failures are sometimes attributed to the oil, but that is almost never the problem.

I’ve explained in previous issues about breaking in engines and will outline it again briefly below, but I also would like to add some important information about what needs to be done BEFORE you start to break in an engine. My experience is that engine problems on break-in are usually due to three things:

The first and most common is that the engine was not properly cleaned out after all the machine work was done. In a production shop the machinists may just blow a little compressed air into the oil galleries and leave it at that, but you should not. When you get your engine back from the machine shop, use a rifle bore cleaning kit and light oil to get rid of any grit or machine shop swarf. Keep cleaning until the white cotton cleaning tips come out white. Otherwise sharp metal particles may either block oil from getting to where it needs to go, or will go to the rod and main bearings and ruin them.

The second thing to do is slather the cam (especially on a flat-tappet engine) with a good quality cam lube. I use Isky Cam Lube.

The third is fill the oil pump with oil before installing it. Otherwise, you run the risk of it cavitating in an air bubble and not pumping vital engine oil at start-up.

Your 1970 Chevrolet small-block is a flat-tappet engine, so be sure the oil you use during break-in and afterward has at least 1200 ppm of ZDDP in it to prevent scuffing and cam wear. On small-block Chevy engines such as yours I fill them with oil and then use a tool available from auto supply stores that you chuck into an electric drill that allows you to spin up the oil pump by going down through the distributor mounting hole. This gets oil up into the small passages in the block and valve train before you hit the ignition.

To break your engine in, start it and run it for at least 20 minutes so it reaches its proper operating clearances, and then drain and examine the oil and filter. There should be no metal shavings or other contaminants anywhere. Put in fresh oil and a fresh filter and drive the car conservatively. Change the oil and filter again at perhaps 500 miles, and then follow the recommended intervals from there.

By the way, your friend is right, synthetic oil was developed during the Second World War to handle extreme temperatures. It will work fine for breakin and general use though it is a bit more expensive.