advice on 1988 300E 105000miles

I have a 1988 300E and I am about to take it in for its 105000 service. It's starting to leak oil badly from its Timing Chain cover. At its last service I was advised that the gasket should be redone for $300. The mechanic (MB Dealer) also told me that it is recommended to replace the timing chain at 100,000 miles for $1100 is this true?

I'm thinking of just doing the cover gasket and leaving the chain alone if there isn't much slap. Is this advisable or am I just being too tight. Does MB really recommend changing the chain at 100,000 miles or is the dealer trying to milk me?

Additionally I'm thinking of switching over to Mobil 1 and going the full 7500 miles between oil services, instead of 3,250(outside Chicago) with Dyno oil. Has anybody done this and did you have gaskets start leaking ?

Thanks,
Dino