Been using 15-40 Brad Penn, Just put the engine back togetherand went with .001 additional clearance on my rod bearings. Was just going to pre-lube and look things over with my valley plate off. Only oil I had on the shelf was 20-50. Stuck my high powered rockwell air drill on the pump and when it built pressure it will about twist the drill out of my hands. I have to wonder what the actual load on the pump shaft is with the 20-50 compared to the 15-40 or even 10-30. I am going to see if the engine will hold good pressure with the 10-30 hot.

Hard to say when it`s hot but I agree w/you on the hand drill cold oil...........it`s syrup thick and I run eighter 5-30 or 10-30 in my car that`s heavily street driven w/bearing clearances bet. .023-.027 w/out issues.

When I first stated dyno testing motors I would see a HP loss with cold oil using 30 WT Valvoline HD oil, motor would make 460 HP 5900 RPM with 130 F oil temps, no other changes the motor would make 475 HP at 5900 RPM with the oil at 180 F with no other changes I now use 5W 20WT or 5W 30 WT Valvoline HD oil and see no HP increase or loss between 100 F and 200 F I do see a pressure drop from 45 Lbs at 1100 RPM with 100 F oil temps and at 180 F oil temp I see 25 lbs at 1100 RPM The car is a lot more constsitent at the track with the thin multi weight also

I find this graph informative when talking viscosity. Especially temperature variation and multi-grade vs straight weight oils.

Look at the example of the straight weight oil at room temp!

Yep, at 20 below an engine with 40 wt oil probably won't even turn over. Need a block heater to get the oil warm or else that baby is stuck.

In engines using hyd cams, I use 20w50. Hyd. system seems to prefer this (makes more HP).
In solid roller or FT stuff I use 10w30 and make all dyno pulls in the 160° range.