Originally Posted by Bror Jace

I wouldn't use any of the store bought cleaners. As a class, they are better than they used to be but if you use one with too much solvent in it, you might get a bunch or wear and/or a seal problem.

I'd use a 15W-40 gas/diesel motor oil to clean up an older or well-used motor. These fleet-type "Heavy Duty Engine Oils" have a very stout detergent/dispersant additive package to keep diesel oil crankcases clean after many thousands of miles. They also have more anti-foamants and more barrier wear additives which make them suitable for high-RPM applications. The weight is even a pretty good one for most high-performance engines.

Months ago, I would have strongly urged you to use Pennzoil Long Life 15W-40 as it had nearly 200PPM of moly in it but very recently, they took the moly out.

Chevron Delo400 or pretty much any one would work fine, I'm sure.

Oh, and not all straight 30 weight oils are non-detergent and ATFs don't necessarily have the high detergency many people think they do.