If the weight is the same, it is perfectly okay to run diesel motor oil in a gas engine. Most diesel oils are gas-rated also. Over the last few years the govt has forced oil companies to take much of the zinc (an anti-wear agent) out of gas engine oil due to the possibility of it contaminating catalytic convertors if the engine burns oil. Not a big deal on later-model engines, almost all of which have roller lifters. Diesel oils still have zinc, aka ZDDP. If you search a little bit you can find Rotella 10w-30 non-synthetic. Standard diesel oil is 15w-40. Definitely on an older or classic car you should run diesel-rated oil these days. High-mileage oils have a little more ZDDP in them than regular oil but not as much as diesel oil. 5w-40 Rotella won't hurt anything if you want to run it.
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