By design this engine should hardly see any vacuum in the intake manifold. But, as noted earlier, if the intake air cleaner is clogged, the higher the demand for air (such as at higher rpm), the manifold vacuum will increase.

Another peculiarity for this engine is the vacuum pump arrangement. On some cars it discharges to the intake manifold, and if vacuum increases when it develops an internal leak, it will suck the oil out through the vacuum pump. Later models might not have had this feature, as the vacuum pump went through a series of design changes.

Same with the crankcase ventilation system. It vents from the top of the valve cover over to a cyclone separator mounted on the intake manifold. The non-condensable gasses are supposed to be stripped from the oil droplets there, with the oil droplets draining back to the sump and the gasses (blow-by, for the most part) going to the intake to be "recycled." Once again, a clogged intake manifold and you can see high enough vacuum at high speeds to suck all the oil droplets into the intake manifold.

Seems clogged intake manifold is a common theme above. I would take the air cleaner housing off and examine the intake manifold as well as air cleaner. You may have had a rodent decide the place was home if the car sat for any length of time.

Any signs of oil leaks? I have had some issues with rear crank seals on these cars, as well as one of the power steering or air conditioning mount support bracket bolts. It got loose and let oil spew out the front of the engine. I was sure it was the front cranks seal, and it turned out to be a bolt that cost me a steam cleaning and a buck.

Also, at higher speeds these engines use more oil, but not a quart in 400 miles. That is excessive. You are pumping oil somewhere you don't want it.

As for gasoline as a lubricant, all things are relative. In ship propulsion shaft bearings, sea water is used as a lubricant. In its liquid, pressurized condition, such as in a fuel pump, gasoline can be used as a coolant and lubricant (it can generate a hydrodynamic film), but as a spray on material it is going to evaporate and leave behind whatever additives are in the mix. Those items may or may not be slippery. I imagine not. And anything designed to be lubricated by an oil will die a quick death if you substitute gasoline.

Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)