> I've heard that it's not a good idea to change to synthetic oil in a
> high mileage engine.
> Any thoughts on this?

Well, if your gaskets are old and only holding oil in by virtue of
waxy deposits formed from the dissolved hydrocarbons present in dino
oil, then yeah, best not to go synthetic. Why? Not sure. I've heard
two competing theories:
1) Synthetic will tend to dissolve those waxy deposits which were
actually acting as a type of stop-leak sealant. If the gaskets are new
enough, they'll eventually become saturated with and softened by the
synthetic oil and be "rejuvenated", sealing the leaks. Gaskets thhat
are more waxy deposit than gasket material, however, will just leak.
This is the more plausible explaination, I think.
2) Synthetic is "slipperier" than petroleum oils, and thus more easily
"slips by" the looser tolerances on an older engine, thereby leaking
out. This one simply makes no sense to me, as I was under the
impression that "slipperiness" was another word for "viscosity", which
varies depending on the grade, not its synthetic or natural origin;
but Autozone put it on their web site, so I thought I'd repeat it for
amusement's sake.

I ran synthetic in my 167Kmile bottom end (80Kmile on new heads since
'97) and never saw a single leak. Of course, I only got about 2200
miles out of it before it croaked of a broken head stud, but there
sure were no oil leaks.