FWIW, I have run several different oil weights in my 331 with no ill effects. The bearing clearances were approximately 0.018". After initial startup with 10-30 mineral oil I performed an oil and filter change after 20 minutes of running to get rid of any junk that might be floating around. I then ran 10-30 mineral oil for about a thousand mile before switching to mobile 1 10-30. This motor was just removed at 122,000 miles due to a broken camshaft to gear pin (the exhaust valves were bent very slightly so I don't know if the pin broke because of the p-v contact or the other way around, stock AFR springs with 80,000 miles?). In the 121,000 miles since the mineral oil was last used in it, I ran mobile 1 10-30, 15-50, and for about the last 60,000 or so miles 0-40 (I get it for free at work). Either way the short block looks great no real signs of wear on the bearings and I totally plan on using this short block with a different cam.
347, AFR 185, FTI cam, Systemax, shorties, 2.5'' exhaust, stock airbox, 70mm tb, 75mm MAF, 24lb
387rwhp, 397rwtq, email@example.com first passes, more track times soon