Buick specified different main and rod bearing clearances depending on the year and engine, but they are tighter than just about any other engine of their time. The typical specification is between 0.0007" and 0.0015" on mains and rods. That is really tight, even by modern standards. Yet Buick specified 10w-30 engine oil. It all works fine if you follow Buick's specified bearing and oil pump clearances, restrict the oil flow up through the pushrods because the new lifters flow too much, and make sure the lifter bores are not worn out. It'll make 10 PSI at 600 RPM and 40 PSI above 2000 RPM with 10w-30 and run for 200,000 miles. That's how Buick built them and it worked, so just build it like Buick did.
Wrong, it is a 5 weight oil with viscosity improvers to keep it from thinning when hot. Those break down and it becomes closer to a straight 5 weight oil over time and use.
