I’m running 15w40 Rotella (non-synthetic) in my 509s - so far so good. I may switch to Mobile 15w50 this season if I see temps creeping up.

So thinner weight oil doesn't have the same capability as a thicker oil to remove heat from the oil or thicker is better at maintaining a lower temp? After lets say 3-5 minute WOT run then down to idle, what oil pressure would be considered "too low" ? I'm assuming this would be a determining factor in selecting the proper weight?

we run t4 non synthetic rottella our active has a 540 we turn 7 grand engine is spotless inside cam and solid rollers do not show any skidding

That was something I forgot to ask. I have heard that it's possible that a roller lifter will skid using synthetic oils.

Yet another oil thread! I'll bite anyway. I can give real world experience on M1 as that is all I ever ran in any of my boat engines. I had a 400+ hour 496 bend a rod when it hydrolocked a cylinder due to missing shutters in the y-pipe. I asked the builder how the internals looked and he it was spotless and bearings looked new. All I ever ran was M1 15W50. Broke the rebuilt engine in on VR-1 then swapped back over.

Mains are too tight on both of those, if you do run it that way, stick to 10-30. I wouldn’t but it’s not mine.

From another thread, You wouldn't run his engines the way they are anyway, so I think the point is mute.

IMHO: If a marine V8 I/O is clearanced for less than 15W/40 I'd be rethinking things. My old as a dinosaur 02.

Somewhat related question, hypothetically speaking, if a good running motor (not blown up, not damaged) is refreshed with new stock size bearings, is it reasonable to assume that the prior bearing clearances remain the same as they were before?

The bearings are manufactured to a given tolerance. So the clearance may or may not change with just a change of the bearing shells.

BRAD PENN = KENDALL
When Kendall closed the doors it was reopened as Brad Penn. Still made from Pennsylvania crude. Has nothing to do with govt this & that. Since the oil is marketed in a different way it can have the old levels of zinc, sulphur, etc. It's good stuff. Still has the Kendall green color to it.
As far as pressure goes, in this case, going to a thicker oil will not increase your oil pressure. Max Oil pressure is determined by a relief valve in the pump. (If the motor had tons of hours and bearing wear on it and couldn't attain the relief valve pressure a thicker oil would probably increase the oil pressure.)
Lesser clearance on the bearings allows a thinner oil to be used. On the dyno oil temp rarely reaches temps that it will in a boat out running hard. The 10w30 might be fine with a good cooler. Or it may not. Hard to tell. Where your engines are at for clearance you should be able to run a 15w40 or 20w50 with no issues. The only question is if you need to or not.
Personally I run Delvac 1300 15w40. I figure if it holds up in over the road turbo diesels it'll easily hold up in a BBC for 50hrs. I've been running it for years and no issues.