Most people think the FSS mileage is TOO high. If it caused you to bring it in more often, I would consider THAT a rip-off.
I would recommend, if you are going to do the oil changes yourself, to go no less than 5000 miles and no higher than 10,000. But 8000 would be about right. It does have a fairly large oil capacity at 8.5 quarts, and also if you use the synthetic, 8 to 10,000 should be pretty safe.
Bear in mind several other items which should be done on a regular basis, such as tire rotations and brake inspections, brake fluid flush (every 2 years) coolant flush (every 3 years), the climate control dust filter every other oil change (per MB anyways). The engine air filter and fuel filter are done every 60,000 miles.
The spark plugs are replaced on this every 100,000 miles or 4 years (5 years on 2000 and newer), so if your 99 was built in 98, you may need spark plugs this fall. There is a label with the production month and year on a label in the drivers door jamb, near the latch striker on the body side.
Be sure to check the spare tire pressure once a year also, I set them at 40 psi, you can always take some out, but not so easy to inflate it to proper pressure on the side of the road.
It's really important to properly care for these cars. If you maintain them like cheap junk, invariably they will end up cheap junk.