oil in my grand cherokee

By far the best choice is a synthetic oil. Pricey, but the molecular make up is so that it doesn't loose it's viscosity like conventional oils. It's just a better lubricant.
That said, I use the Wally World brand. It has the same rating as other major brands and is a good bit cheaper. I only have one vehicle that is under 200K. The secret is to change the oil at regular intervals. 3000 miles is the norm. Manufacturers recommendation is higher. If you work it hard, then you may want to go with less miles between oil changes.
The best thing though to keep your oil in good shape is the filter. I will let you do you own research as to what filter you want and should use.
Two things are detrimental to oil life. Heat and contamination. With a gas engine you can't help but to get contamination. No way around it. so you want something that will filter it as best as possible. A good cooling system also helps the oil life. Run it hot, and run it hard, the oil will not last as long as it would say if it was 95% highway driven.
Hope this helps
 
15/40 rotella
 
If you have any leaks at all the syn. will make them bigger leaks.
 
V8 or I6?
I wouldnt switch to synthetic with close to 100k on if its a V8. Even if there is no external leaks on a V8, it probably uses oil (just my experience with the Chrysler V8s).
 
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