yusych

I just reciently got my 1991 Ls400 W/53k. About a week ago i used engine flush and i changed the oil, i put synthetic Castrol i think it was 10w40, and i dont know if the last owner used synthetic or not. But anyway i recently discovered that i have a main real seal leak about 3 days ago. I have been reading the forum on this topic and i have found that some people used some engine additives or stuff like auto-rx to try and stop the leaks. Now my question is is there anyway of fixing this problem, or is replacing the seal is the only way to fix it. And another thing should i continue using synthetic or regular oil, and what brand of oil seems to work the best with these cars, thanx