This has likely been thrown around for a few years but I was wondering if I should switch to synthetic oil this fall as winter is around the corner. The truck is an '01 auto and I always plug it in when the temp. gets below freezing but I was thinking it might be a bonus to switch. I am and have been using Rotella dino 15w40 for years but the question IS what weight do you use when you switch to synthetic? I know that the #'s are different with this oil, like I use Castrol 5w50 syntec in one vehicle I have and I know I wouldn't use that in the Cummin's but what is available for our trucks?

 

Always good idea to switch to synthetic - and not just for the winter (in MHO).

You'll get lots of arguments as to which is best, etc. Do your research to satisfy yourself. Remember that "synthetic" is a rather broad term and can be applied to group III (highly refined dino), group IV (POA) and group V (esters).
Which type is best? ...depends on what you want to accomplish.

Some synthetics flow into the -50's - so you'll have no problem turning the engine over - even without the block heater plugged in.

Where abouts are you and how cold does it get? That may help you answer the viscosity question - remember that a 15w-40 synthetic will still be flowing at temps where a 15w-40 dino has long dropped below it's pour point.

 

Where I'm at temps can get -20 or lower. I run synthetic and my truck will still turn over like it's summer time.

 

I live in eastern Canada and here the temp. usually doesn't get below say 10 f too much, maybe a few days a year. I just was thinking as the truck and batteries are getting older it might be something to think about. Thanks.