slant 6 oil weight

It is not much different than any modern engine. Indeed, the 3.8L in my 2002 T&C looks very similar to my 60's V-8's (same pushrods, timing chains), so most oils would work. Our flat tappet lifters need as high a zinc content as you can get. I recall some diesel oils (CJ rating) have that. Search since people have talked about that, plus additives. Diesel oil is good to better suspend the black carbon, if your engine is worn (high detergent). A thicker oil will help seal leaking rings. There are "high mileage" oils in the store. For best mileage, a thinner oil helps, even the new 0W20 synthetic.