7.3 Oil Change questions

[Thomasbeau]

My TV is a 2002 F-250 with the 7.3. I have owned it since it was new and it’s just shy of 150k. It used to be my everyday driver but now it only takes me to bank on weekends and pulls the Hideout for about a dozen camping trips a year.

Since it was new I have always used Rotella T4 15w-40 in the engine but in the last year started using Mobil Delvac 15w-40 and the truck seems to like that as well. But since I don’t drive it much anymore I am doing a lot less oil changes. I think I would like to switch to a full synthetic but I’m not certain when it comes to weights. If I switch to Rotella T6, should I use the 5w-40 or the 15w-40? There is a difference in price but since I only do two changes a year now I’m not concerned. Any opinions, is this the direction I should be going with this engine?

Also, I was planning to try out the Archoil on my next oil change but if I go synthetic is that a waste of money?

Thanks,