Dude, no- the MityVac won't work well for oil changes. Go out and buy a Motive Products fluid transfer apparatus available from Performance Products 4 Benz. MityVac doesn't even work well for brake bleeding. Only thing MityVac works well for is diagnosing vac leaks. You'd be better off changing the oil the old fashioned way- while hot and draining from the plug on the oil pan- because you get more of the old oil out that way. But the difference between the two methods is slight at best. A dipstick oil changer certainly is convenient.
Very nice SL. And I mean very nice. My recommendation for a cover is you should have built a garage before buying it. No SL should be forced to live outside. Get an MB cover or one from Performance Products in the meantime until you sort out garage accommodations.

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'02 BMW 325i
'85 300D 450k
'93 190E 2.6 170k(killed by tree)
'08 Ducati Hypermotard 1100S 6k
'06 Ducati S2R800 14k(sold)