For instance, can you change your oil every 5000 miles instead of every 3000 miles if you use synthetic?

 

Yup, that's it and if you drive under extreme conditions you benefited from using full synth than that of dyno oil . I am using full synth at recommendations by Jag mechanic .

 

Why are you changing your oil every 3000 miles?

 

I just got the Jag. I changed my Mazda's oil every 5K miles using synthetic. How often do they reccomend you change an XJ40's oil?

 

The maintenance interval for the XJ40 was 7,500 miles. If you are going to use full synthetic, I would be content to go the full 7,500 miles. The later cars use 10,000 mile intervals, and personally, I'd be OK doing that with the XJ40. But some folks can't bring themselves to do that. I run full synthetic in my XJR 6 cyl., and I go 10,000 miles between changes.

Your choice!