Light viscosity break-in oil

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May 14, 2003
What is the reasoning behind using lighter viscosity oils for breaking in a new or rebuilt engine? I will be putting the fire into a rebuilt Pontiac 455 and was wondering what viscosity of oil (and brand) to use for break-in and when to switch to a higher viscosity if you are suggesting starting low. Any opinions are appreciated.
I have built a few SBC's. I have always used a conventional 10w30 for break-in. I don't know why someone would use a lighter oil at break-in.......Perhaps so it will flow faster at initial startup. I always just use alot of assembly lube on the cam and lifters and prime the oil pump just before startup. I then switch to a synthetic at about 3000 miles (I change the oil several times in the first 3000 miles with conventional oil though). I have never built a poncho, but I don't know why it would be any different.

[ July 10, 2003, 07:38 PM: Message edited by: sbc350gearhead ]
Thanks Gearhead. I would think that if you prime your oil pump before starting it and get oil through the entire system, you should be OK with 10W-30 or 15W-40. I am not really sure though. I have seen posts where people are using 5W-30 for initial startup and the next oil change. Any other thoughts people?
The reason used to be that the added friction and more metal to metal contact help the rin/cylinder seal by ahoning type process. That was my understanding.

When rebuilding an engine that stimm may be somewhat necessary-But I really don't have the practical experience to know. Hoefully another engine guilder to two will surface.

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