I just bought my first car, a 89 accord lx with 150k.

I'm not sure what oil was used in it but I'm going to change with syn, mobil1 high milage. (what do you think about this oil?)

I've heard this may cause leaks if conv was used previously. Where would these leaks be and what gaskets would I have to change if they did leak?

Also, where's the best place to find gaskets, and how would I change them?
I've heard of liquid gasket products being used, but are preformed gaskets better, more costly?