I need to know by tomorrow (fri. 5th). I've tried searching to see if there is anything negative about doing this but come up with nothing. I'm going to the track for the first time tomorrow. My motor has around 5k on it since the build. After break in, I've used nothing but Mobil1 5w-30 syn. I bought 3 quarts mobil1 syn 5w-30 & 1 quart Valvoline VR1 20-50. I was thinking about trying this. Using the majority ( 3 quarts 5w-30) which is thinner & easier to circulate through motor & 1 quart of the VR1 20w50 for protection at high rpms & the rigors of the track. I just need to know if there's anything wrong with mixing oils or is the weight/viscoscity difference will screw things up. Looking at it from the oil's benefit's perspective it seems like a good idea but scientifically & the facts, I don't have a clue. Is this a good idea, bad idea? Does it matter? Will it hurt anything? I figured the fact i'm using 1quart to the 3 quarts of 5w-30 will hopefully balance things out. Probably something I wouldn't consider for the winter time.