Delo 15w40 because it's got higher zinc content which seems pretty important to old flat tappet engines, at least the Cadillacs. In a Cadillac car you can run 10w30 just fine, but it's seeing half the RPMs in front of TH400 in a Coupe that it is in front of an SM420 and 'slow and slower' two speed rear axle. I tried the thinner weight, modern synthetic oil in the previous engine that failed. So, I went back to the heavy duty diesel engine oil and that's working so I'll keep running it.
