I just bought a 2001 s type with 121k miles and the owner put full synthetic in it but its been on conventional oil since day one from doing research on the 1st owner so now im worried about leaks and issues. Can i go back to conventional or at least synthetic blend?

 

Switching between synthetic and conventional oil is perfectly fine. You also don't get leaks from using synthetic oil.

What you asking isn't a concern on any car made from early 80s and up.

Personally, I prefer synthetic as it has wider temperature range - it flow in cold and doesn't cake easily when too hot. However, regular oil is just fine if you change it frequently and everything is working as it should.

 

There's no requirement to use synthetics in an S-type and no known real world benefits. Nor is there any need to change the oil at less than the manufacturers prescribed interval.

On the other hand they will do harm.