Synthetic lube and leather seals

If that is an original seal, it has served you well, replace it. Synthetics drip out of even the best seals if they have run anything accept synthetic. If it is original, what were your thoughts pushing around an old seal like that? Don't mess with the success of a 40 year old seal.

Your best bet is to stick with the oil you started with fresh seals. If it was dyno oil, run dyno oil, if it was synthetic, run synthetic. Don't cross pollenate. Try draining it out, cleaning everything with carb cleaner and put dyno back in.

Good luck.