Thats really good information which answers my question.
This engine has a main bearing bore of 2.250" clearance of 0.00275"
Rod Journals bearing bore is 2.001 with clearance of 0.002"
I guess that gives me on the mains approx 0.0035" when hot which would indicate at least a 40wt
oil. I assume if you wanted to use a really thin
oil for less friction and more hp you could go to an oversize bearing shells and reduce clearance.....