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Choice of mineral engine break-in oil

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Hi

I am currently building a CA18det engine, the recommended OEM oil is 10W-30; 20W-40; 20W-50

which mineral motor oil should I choose for break-in?

I intend to increase my bearing clearances because the engine will run at more than twice that power but will maintain an engine speed limit of between 7000 and 7500 rpm

Whichever one allows you to maintain oil pressure at the oil temperature seen during break-in. If in doubt, go with 20-W50. If your oil pressure is too high on initial startup, then switch to a lighter oil.

ok thank you David, how do you know the original oil pressure of your engine?

Experience? Factory Workshop Manual? A rule of thumb for some engines is 10 psi for every 1000 RPM. But we run our Mazda MZR race engines (stock OEM bearings / clearances) at 8000 RPM and am happy with 35-40 psi hot.

If the Oil Pressure is greater than about 120 psi you risk failure of the oil filter, which makes a big mess let me tell you.

ok, thanks again David, I will start by looking in the workshop manual