After the suggestion I gave you about your sister's car, I realized it applied to me as well. So, I've decided to become a "data point" myself. I put in 4 qts of Mobil 1 15W-50 at approx 159k miles on 02Oct02, with a standard ACDelco DuraGuard filter. I'm putting about 400-500 miles a month on the car, & I'm not going to change the oil until I get to 170k miles. Time is the unknown here, & at my current rate it'll take me at least 18 months to get there. I plan to do an analysis then. At this point, I think I'm going to top up as needed with Delvac 1 (for the extra additives), but if anyone has any compelling arguments for staying with 15W-50 I'll consider that too. I haven't decided whether or not to change filters 1/2 way. Again, I'll consider any arguments for/against. Of course, other things can change my plan. My son starts driving pretty soon, & he'll be using my car. That may alter the amount of time it takes to get to 170k. If it takes less than a year, I'm not going to bother with analysis. I'm only at 160.2k now, so unless I change jobs I don't think that will happen. The other big possibility is an accident. If the car's in an accident, it may negate the test. Or, if I'm riding my bike & that bus with my name on it comes along, no one else in my family cares about my oil neurosis & the test's off. So, anyway, I wanted to let you know I wasn't trying to be a smart-aleck. I still think your sister's good for 10k-12k miles with her driving conditions & M1 SS top ups. Hopefully, I'll have a good outcome with mine too.