This weekend, finally gonna around to doing an oil change and thermostat.

I want to go with a full synthetic, which one? and what weight?

was suprised by the many choices, 5-40, 10-30, 15-50.

I do mostlyl highway, above 2k rpms if that makes any difference.

 

My feeling is they're all about the same. Your main consideration should be how easy it is to acquire one brand over another. Having to mail order a qt when you need to top up can be a PITA.
Personally I'd use 5w40.

 

I run amsoil and really like it.

Their 5w40 is good stuff.

 

5w40 is too thin IMHO- I have been running Mobil1 15W50 since I bought the truck (@42K miles). Not all synthetics meet diesel requirements. Mobil 1, Rotella, Royal Purple, and Amsoil meet diesel requirements. Figure out which you can get locally at a reasonable price. (for me, Mobil 1 and Rotella) And for a 12V you want the older CH spec (CH4 I think instead of CI4)- the newer oil spec works better for the newer engines with roller cams whereas we have tappets (mushroom shaped solid lifters).
Cummins and Dodge spec 20W40 for the 12V's unless your in extreme cold weather. I wouldn't drop below 15W40- if the oil is too thin you'll have issues, like low oil pressure.

There is another thread somewhere on here regarding oil and the which for what that has more detail on the ratings. Read the OM in the glove box and find one that meets those requirements. If you google SAE oil ratings you'll find the rating information straight from the SAE.

 

hi
5w40 here rotella syn , have not seen any diff in oil psi over 15w40 regular oil !

 

Any of the previouslly mentioned syn oils will work great for you. Avaliability and price is the issue. If you can purchase a high regarded brand at wally world for $16 / gal or $20 from a sales rep that moves around or ships, there is not much to consider. In the end, the engine will never know the difference, no matter what. You do not have engine break downs from lack of quality. Use quality filters also.

 

I switched to Amsoil 5-40 Diesel synthetic and I love it, I get instant pressure even in -30C temps, also the truck starts without being plugged in like 20 degrees colder then it did before. Saved me once at work, wasnt planning on staying there too long so i didnt plug it in, it was -40C out without the wind, sat for 10 hours and it started. I couldent belive it.