Ok, so I'm currently using Rotella 15W40 oil in my car, but it's a quart short and I need to top it off. Is it ok to top it off with 5W40 (since it's getting cold here)? Or would that mess everything up since the latter is thinner than the former? Would it be better to just do an oil change if I want to use the thinner oil?

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1983 240D, 272K+ miles (odo broke)