Recently got a 2006 Toyota 4runner with the v6 for my girlfriend. The 4runner was in great shape with 179000 miles on it. Guy had it since new and only ran conventional oil in it. Right when I got it I changed the oil to Shaeffers supreme 9000. Now 3500 miles later the rear main seal on the engine has started leaking. When we got it it was completely dry underneath and not even a hint of a leak. I've always herd to not switch to synthetic on higher mileage because it will cause leaks but I kinda just thought that was an old wise tale. Is it just a pure coincidence or do I need to go back to conventional oil in this?