as far as I know the old diesel motors prefer mineral, they were made before the synthetic oils came out. Think of those old fashioned rockers tapping the valves, flat lifters, push rods, they need viscous lube.
I too, use the penrite 20-60 extra zinc in my 2h. Ran awhile on the caterpillar, delo and shell 15-40 s but after much reading went back to mineral oil with zinc. The 20-60 suits Australia due to being a warmer climate. Don't see snow too often...It is a bit more exy, but I really am aiming at high milage from the one engine in every way possible.
The synthetics have detergents. To clean an engine with mineral, you have to change the oil more frequently, with good long runs at over 2000rpm, to minimize carbon. And as Rosco said, injectors in good check.
Read an article about redline oils cleaning an engine with 3x oil changes in an hour!
yeah been talked to death