My Duster with 3.91s runs about 3500-3600 rpm at 68-70 mpg. Usually I will run around 3300-3400 rpm on the highway, but sometimes you need to run a little quicker to keep up. Eventually a 4 speed will be going in. Tires are 255/60/15s. Motor is a warmed over stock compression 318 (with XE268 cam) with 45K since the last rebuild. Doesnt use a drop of 15w40 Delo oil between changes. The engine buzzing doesnt bother my ears, but with regular oil changes (every 1500 miles), am I flirting with danger at the revs mentioned? I dont think so, but just wanted to hear some opinions!