I've been running Valvoline SynPower 5w-40 in all my stuff, from my '88 Town Car (302), to my '89 Firebird. No issues, doesn't seem to like it or hate it any more than whatever the PO filled the crankcase with but it makes me feel good. I also use Motorkote at every oil change now. I use Wix oil filters, the ones for extended drain intervals yet I change my oil every 5k. Oil is cheap and easy to change. Even when it was -10f a few months ago, it only took a second before I saw the gage register pressure..
I have used Restore, that stuff is super goopy when cold. I've seen videos where it does indeed increase compression. I tried using it in hopes to reduce oil consumption. It might've helped for two tanks of gas but that was it with the old 350 in my truck. I doubt I'd use that again unless I knew I had low compression and didn't care about the engine. I used it in my lawn mower to see if it would help that with oil consumption. When I checked the oil before the next cut, I noticed metal shavings and subsequently, rod knock. I changed the oil, put Motorkote in and while shavings are still present, no knock under load. I'm sure it'll die eventually but at least it doesn't sound like death any more.
Oh and 15w40 isn't any thicker than 10w40 at the same hot operating temperature. I wish people would stop repeating that nonsense. However, 15w40 is indeed thicker than 10w40 when cold. That's the difference. The "w" stands for winter; that at a specified temperature (32f maybe? I forget what the spec is.) the given oil BEHAVES like a straight 15 or 10 weight oil would. So make no mistake, those oils will be plenty viscous when cold.
