We use Rotella T 15-40 in ours....have been doing this for years in our Direct Drive
Butlers in GMC Vans
We also change out the oil and filter way early some could say at 1000-15000 road miles...and way before the extra miles would add up from running the
Butler (or direct drive) to even total 3000 miles.
We have vans running with tons of miles and tons of hours....one right now cleaning part time (a 93
Butler and GMC) has 250,000 road miles and over 11,000 hours on the
Butler unit for a total of about 613,000 miles on that original motor....
So, for us....oil is cheap!
Keep in mind, you are ingesting air with dirt particles into that motor, so we find changing the oil more often, less dirt to cause abrasion and wear as well....
Just our philosophy and it has proved successful for a long time.
Rotella T is not a synthetic, but when my racing friends dyno test motors, they find the properties to be outstanding.