I have a 97 6.5 with 4:10 gears. Which oil, regular or syn? wt ? I tow a min of at least 3 times a year, 10,000 lb 5th wheel.
Thanks

Synthetic is always better whether you tow or not. The only advantage dino oil has over synth is that it's cheaper.

Use what ever you are comfortable with. Synthetic is generally better and provides longer change intervals.

Use what you like,but I have to tell you if you use DELLO 400 15/40 you will get the best of both worlds .I have not seen any synthetic that works any better and I have tried every one over a 20 year period .in every thing from Diesels ,watter cooled to air cooled american and forgen.and gas engins american and forgen 4 cyl to 12 cyl ,stock and high performance and from redline to amsoil and just about every thing in between .what i found is ONLY at high rpms was there an advantage and only in a hair more hp on a dyno. your best bang for the buck is DELLO 400 15/40 change it and the filter every 5 to 6 thousand miles and use a Good filter .at $1.50 a quart (at COSTCO) (6) 1 gal jugs $37.00 you can't beat it in a Diesel for protection and price, if you change it as stated .this means more money for toys . by the way I have a used car lot and we go through a lot of oil .and Phoenix is very hot and rough on vehicles .:lol: