Search the archives for instructions on checking the degrees of stretch to see if it is time to replace the chain. 100k is a good estimate on when to change the chain, tensioner, and guide rails on most models.
Edit: oops, just saw your clarification -- that price is probably OK.
Yes, thin synthetic oil will slip past aged gaskets on an older engine. Hope this helps, Andrew Seidel.
85 500SEC Lorinser
83 300D Turbo