Tonneau cover should make some difference, the tailgate is like a parachute. I always stick with manufacturer info as far as oil viscosity, I don't wanna damage my engine. Synthetic might help a little though.

 

Actually alll the oems say a diesel rated10w30 is kosher. I tried it for the winter in my psd and the cold starts were great. I am sending it off for a uoa soon and I am not expecting any higher rates of wear.

 

Actually, that myth has been disproved many times. There is virtually no difference in MPG with the tailgate up or down.

 

I live in a hot place, so I'm going to use 15-40 oil

 

And I'm supposed to believe that because you said so Sit up against the tailgate while somebody is doing 80 down the interstete and then tell me about your myth

 

driving with tailgate down or off actually gets worse mileage

 

OK well thats better, now that you said it I believe it

 

well I watched in on myth busters the took 2 identical trucks drove side by side one tailgate up one tailgate down the one with tailgate down ran out of gas sooner. used same trucks reversed the test and same result the truck with tailgate down ran out sooner.

 

http://www.sema.org/ltaa/do tonneau ...ove gas milage Cant get the link to work but SEMA did a study and tailgate down got slightly better milage, tonneau cover got slightly better yet and a aerodynamic cap got the best. There was only like 1.8 mpg between best and worst. Mythbusters used 2 different trucks.

 

check this out