Prior to switching to synthetic, run 2 trials of AutoRX. See
www.autorx.com and the additives formum of
www.bobistheoilguy.com.
This product is only available over the internet and gets rave reviews on BITOG.
I used it prior to switching to synthetic oil.
It will rejuvenate the seals, clean the combustion chamber, and clean the ring packs.
It strikes me that you are a logical candidate to use AutoRX, given your mileage and lack of knowledge of how the vehicle was maintained prior to you owning it.