possible oil loss sources.
1) Faulty piston ring to cyl wall seal.: Seems highly unlikely as I cannot imagine your car having that many miles on it. my 240 has 337,000miles and is only now starting to show ring seal pobs after I boosted cyl compression significantly.
2) External leaks: Blown seals such as the crank seal may cause this. One would think that this would be evident though.
3) Worn valve assembly.: This is a possibility although not terribly likely. I rebuilt the head of my 240 after 330,000 miles and the valve seals were still in moderate shape, not great but survivable. Although uper engine wear due to improper low grade fuel may contribute to accelerated wear...again I think unlikely.
4) Clogged Crank Case ventilation system: This sounds like the most likely of culprates to me. this could cause the crankcase to become overly pressurized and to force oil back into the cumbustion chamber where it would be "consumed". I say that this sounds likely as many people never think to service this portion of cars, and infact was the issue with my particular car. indications of this would be an oily throttle body, sparkplugs and oily blueish exhaust. I would follow the suggested service of the crankcase ventilation system for your particular car and see what happens...if that doesn't solve it you really haven't lost too much cash or time and the service should be done regularly anyway. Don't forget that it could very well be that you have several of these sources as you have indicated that the leak is what I would describe as sevear. good luck