Its time for a oil change and I would like to use synthtic oil but have a few questions. I have heard various reports about synthetic oil and would like clarification. First, I've heard that synthetic should only be used in new engines as it shrinks the engine seals. Also, I've heard that once synthetic is used then conventional oil can't be used again. I've been using Mobil 1 synthetic oil in the '68 Jaguar and would like to do the same for the 944. I'll probably keep the car forever so I'd like to use the best oil possible

 

Try doing a search on it. It's been discussed in quite a few threads. As for the seals I think the problem is that synth oil is thinner and if the seals are old and a little worn the oil can leak out.

 

There's no doubt that synthetic oil is better and provides longer protection than conventional oils. You can use conventional oil after using synthetic and vise versa - I have done this and no problems occurred.

The only problem with synthetic oil and older engines is the seating of the piston rings after an engine rebuild. The engine must be broken in with conventional oil and the rings seated before the use of synthetic oil. Usually takes around 1,000 miles (though Total Seal Rings recommend 3,000). If synthetic is used from the start, the rings will never seat, and you'll burn oil until the next rebuild. Some new cars today like the Corvette come with synthetic because the block honing process is far superior on these motors than a few years ago, and the rings can seat immediately.

I use synthetic in my 69 AMX's engine, transmission and differential. Will be putting it in my new/old 944 soon.