I am curious as to the results of mixing different grades of oil. I have searched but could not find a formula or a calc for determining the resulting grade after mixing oils. The following makes sense to me, but I want to make sure it is right. % of xW1-G1 + % of xW2-G2 = xW-G (xW1 * % of total) + (xW2 * % of total) / 1 = xW (G1 * % of total) + (G2 * % of total) / 1 = G Using the above equation, 3 quarts of 5W-20 and 1 quart of 15W-50 would yield an oil that is 7.5W-27.5 (5W * 75%) + (15W * 25%) / 1 = 7.5W (20 * 75%) + (50 * 25%) / 1 = 27.5 Is this correct? Am I calculating this correctly, or am I leaving something out? Thanks!