Utah-kat, I think your parts guy is doing you a disservice. If you change oil according to the FSS (I assume you have on on your '99) you should get almost twice the mileage out of you oil than you do now. Just make sure you use the MB fleece filter if you do. They changed to the fleece filter because the paper filters couldn't last as long as the oil did.

It would be cheaper to buy the fleece fileter and change oil according to the FSS, (which is usually at 9,000 - 12,000 miles for me) than to use the paper one and change it at 5,000.

Of course, if you don't have the FSS then it's every 7,500. Carry on!