this weekend, i'll be installing the german castrol 0w30 oil into my m50 (aka jesus among synthetics under $6USD) and see how she fares. the car will be used as a delivery vehicle starting next week, so we'll see how long it holds up under severe start/stop cycles before it breaks down according to the Roopnarine method for viscosity determination . multiple tests at bobistheoilguy put new GC 0w30 at a viscosity of 12.2 at 100C, which is just on the borderline for a 30/40 weight (40 starts at 13). m1 0w40 tests between 13.2 and 13.5, so i doubt there's going to be that much difference, especially since both are very good oils. i know that i could certainly stand to stomach a few extra mpg now, so i wont sweat the fact that its a 30 wt. (i'm sure i'm not the only m50 owner in a warm climate running a 30wt).