I myself use Schaeffer's 7000 10w30 now that I am halfway educated by you guys, but a friend of mine has been creating his own recipe. The vehicle is a 2001 Chevy Malibu 3.1L with 4.5 quart capacity. He's been taking 3.5 quarts of Pennzoil 5w30 SL dino and mixing it with 1 quart of Mobil 1 5w30 SJ Tri-Syn in a 5qt Pennzoil jug. I have used both of these oils in the past and actually am responsible for him trying synthetics--what I haven't done is mixed oils like this. He seems to think he is getting the best of both worlds by getting a blend at a reasonable price. I don't know his change interval but I would guess he goes by the old 3K or 3month myth. What do you guys think of this practice?