I have a 98 VW Passat 1.8T ("The Sludge Monster"); been using Mobil 1 since 10,000 miles, I'm now at 85,000. Originally 15W50 (was it 20W50?), when the 0W40 was available I switched to it. I usually change my oil at 5K, took it to 7K once and there were two times the dealer messed up and I ended...