The D is up for its first oil change since i owned it. it has 131k mile on it. plan on using mobil 1 high mileage. my question is what weight i should use. i do drive it as a daily driver with one or more 300-400 mile trips per month. its been averaging 100-105 degrees here during the day. engine is a 5.9L

 

10w-30 ... i personally run synthetic high mileage...

 

the 5.9 calls for 10w30, going fully synthetic just use the recommended weight and be done with it.

 

Not always the need for sysnthetic more of a choice. Just remember once you use sysnthetic you have to stay with it. 10w30 is more than fine to use. I personally am a fan of the valvoline high mileage.

 

this is not true at all you can switch back and forth if you want to and the engine will still work just fine, it IS possible that switching to synthetic can cause leaks, and switching back is not often advised, but doing so will NOT harm anything

 

+1... switching back and forth will NOT hurt anything. although i dunno why anyone would

 

I use Mobil 1 full synthetic every 5000 miles. It turns out to be cheaper in price over a year when compared to conventional every 3000 miles.