I Used to be a moderate mixer but after my last experience I have decided to stop it all together ( well maybe I'll mix M1 15W-50 with M1 10W-30. The brew I used in 3.1 V6 Grand Am consisted of Castrol GTX 5W-30 (3 l) and Syntec 5W-50 (1 l) I used an almost empty (maybe 0.25 l or 8-10 oz. of oil left) jug from Delvac 1300 15W-40 and mixed it all in there. The previous fill was also Delvac 1300 15W-40. About 400 km after the oil change I checked the dip stick and noticed that the oil looked cloudy, so I checked the left over mix in the bottled and it didn't look good I was cloudy with milky chunks of stuff. naturally I warmed up the engine dumped this stuff and filled the engime with Valvoline Maxlife 10W-30. No more mixing for me.