Does a looser build need thicker oil.0030 1-4 mains..0035 #5 main.Rods .0025 Would 10/30 or 10/40 be a problem with these clearances.I thought the oil weight played a part in how tight or loose the build was.

 

10-30 will be fine for those numbers if you want to see a little boost in oil PSI you can use 10-40 or a straight 40W.

 

There should be a relationship somewhat between clearances and oil viscosity. Your ambient climate might be a slight factor also. Your a tad bigger than original, but the oil should probably do O.K. Watch your oil pressures. A more viscous oil will probably raise them some. There are many multi-viscosity oils with a base of 15 or 20 which might be better in a hot climate.

 

Thanks guys...with 20/50 my oil pressure is 65 cold @750rpms.At temp 175 it drops to 40.If I get on it hard a couple times it drops to 30,but recovers back to 40 eventually.Was just wondering about the connection between tolerances and the oil cushing.Thinner oil does have benefits,but if I'm to loose,a 10/40 or less might spell trouble...thanks again

 

A thinner oil will flow better, remember oil does the important job of removing heat from bearing surfaces, in an ideal world you would have an oil just heavy enough to maintain an oil film, keeping surfaces cooled and with minimum load from the oil pump.
Try a 10w40 and see how the oil pressure holds up, if it's no different you might go for a 10w30 next oil change and see how that performs.
If you DO need 20w50 to keep good oil pressure I think you need to consider taking a look at the bottom end sooner rather than later.

Roger.

 

I would like to run a thinner oil for those reasons.10/40 it really should not drop that much.I'd like to know what other builds with wide tolerances use.I have a feeling most are not using 20/50.At least on a street car.