go ahead and mix them. Net effect is the 40 will merge with the 30 and balance out like a 35 (which doesnt exist) but no problem. At normal engine temp. the oil will reach its higher number, viscosity. They it stays at that as long as its clean. When its cooled down, it then acts like the 4 or 5w for a short time as it warms up. Then its back to 30, or 40
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