1. TRUE, but not the rubber gasket under the main covers, instead it is the valve cover vents on top of the main covers.
These are sealed with RTV ( no gaskets ) and the RTV tends to break down over time.
There is a DIY of fixing this, look in below link.

2. Not true, I know of one with over 450,000 miles without anything internal to the engine failing.

3. Mercedes has recently changed the recommended from 0-40 to 5-40 ( Mobil1 ), although any good synthetic is probably OK.
Transmission oil at 70-80K, I used Pentosin.ATF 134. I also changed the transmission connector plug and the conductor plate along with the filter.
Due to age, not mileage, you might also want to replace the brake fluid, PS fluid, and coolant.