Just some guy
Join Date: Jun 2001
Location: SC foothills, USA
This comes up a lot and if you can find them, there are threads with way more info than you want. Zray summed it up nicely. Mobil has taken the trouble to meet specifications with a couple of oils that many vintage Mercedes cars and Porsches require. And put it in writing. Unlike most other brands that hem and haw and rather vaguely "claim" to be good for various older cars. It so happens what is good for those older European cars is perfect for our old Ford engines. The specs are out there and previous discussions of this included a chart. Basically all you need to know is that some of the best synthetic motor oil ever for our cars is the 15W-50 and I just picked some up on sale at Wal Mart for $22 for 5 quarts. What may be the "best" is the Porsche spec 0-40 Zray also mentioned. It meets a higher standard, is not on sale like ever, and is really overkill for my 351W's if you ask me. Good stuff though. The weights are key. They don't sell a different version of 15W-50 that I've seen, if you see those numbers on the full synthetic bottle that's it. 10W-30 and 5W-20 are very different blends intended for different engines.
I have a Japanese engined vehicle that I decided to treat to Mobil 1 10W-30 (correct for 1990's Asian stuff) after a long life of (I imagine) the cheapest oil available. It was unhappy and immediately started leaking small rivers of oil. Mostly from the cam plugs, valve covers, and distributor seal. Never leaked a drop before. That one didn't go too well but new gaskets and seals I've bothered to put on seem to have corked it up. (I still haven't finished changing ALL the gaskets. I don't value the vehicle highly enough.) I've yet to see such issues swapping Fords (new and old) over to synthetic but I keep a sharp eye.
Dirty little secret. If a car smokes blue smoke visibly out of the tail pipe, you can change it over to full synthetic Mobil 1 and the blue smoke will almost disappear about the second oil change. It's still burning oil just as bad, but you and everyone can no longer really see it.