If your car (it's been a long time since I worked on a MW pump) doesn't have the drain plug then use a MityVac or it's ilk to suck the oil from the pump. As Sugar Bear said, use mineral oil as a flush. Refill with a good 30wt. oil. I've never used a synthetic oil in the pump but it probably wouldn't hurt it.

You should have a MityVac anyway to trouble shoot vacuum issues. Good reason to buy another toy, err, I mean "tool" to add to your kit.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now