I don't bother on older engines.  It can cause them to start leaking in places where the old oil has packed into the seals and is plugging up gaps old age bring on.  I do run it in the diffs though.

I'm rebuilding the engine so I'll be replacing all of my gaskets and seals. I went ahead and bought some Valvoline Max life full synthetic because it was on sale for 3.95 a quart and I couldn't pass it up. If it's a bad idea then I'll take it back and get conventional oil, I just figure if I'm gonna put all the effort onto regaining my tired old 64hp it might help it along to use synthetic oil and a premium filter. Also replacing vacum lines and rebuilding the carb. If I have time and money I may re route the exhaust to be a little straighter and gut the catalytic converter to allow for better flow... I'm not trying to build a street racer just a DD that I can put a few more miles on and be trail capable.