the thinner rings like a 3mm or even a 2mm have more conformability to the bore... aslong as you have enough relief capacity in the land for the application and enough tension,, it will be fine

i run a 2mm oil ring with no vac pump and everything is as dry as a bone

for that cab,, id start with 15-18lbs with a fresh hone though,, i doubt those pistons have alot of relief capacity