My thoughts are this.... If you run the car during Hot summer days out on the street/strip.... 20w-50 is what I would run. All depends on what you are doing with this engine? What are your engine specs?
1968 390 2V 68k standard bore
new moly plasma rings, rod, main, cam bearings
Cam Research 226/232 .540/.565 110 lsa cam
Cloyes tru-roller timing chain
Erson rocker shafts and pushrods
Survival Motorsports stage X heads with 2.2” intake and 1.66 exhaust valves
PI intake manifold with thermal barrier coating
428 cj exhaust manifolds
Pro Systems 750 HP double pumper carburetor
Faron recurved distributor
C6 transmission
ATI 2700 rpm torque converter
3.50 traction lock rear gears
Magnaflow 2.5” mandrel bent exhaust with modified stock exhaust tip
I'm in AZ, but I am not here for the summer months. Typical temp in the 70's.
The only thing I have against 20/50 is that on my other car I run 15/50 Mobil 1 synthetic and have an oil temp gauge in the pan.
It takes 6 or 7 miles for the oil temp to come up to its max, oil pressure goes down within a few miles of easy driving.
I feel that I may have more wear in just getting the oil up to temp with 20w50 than I would with the 10w30.
It seems a lot of my driving is testing out something new on the engine, so it gets floored almost every drive.
That being said, I don't drive it alot but I don't want to spin a bearing either.
When I get things sorted out it won't get as much abuse.
I pulled a rod bearing to check after 150 miles with the 30w, and it looked good.
I intend to shift around 5800 rpms.
What am I going to do with this engine?
Run it hard every time I get a chance, that meaning taking it up to 5000 to 5800 rpm's in spirited driving.
It is not a race motor and is not in a race car.
I will take it to the track but only when major changes have been made.
I'm not going to beat on the car at the track.