I've never done it but I know there are places you can send a sample of oil to have it analyzed. If metal particles are found in it, that points to the engine coming apart. If the oil is perfectly clean, that suggests the engine ran out of oil, then some was added later to cover it up. The metal particles will be clumped on the bottom of the oil pan rather than dispersed in the oil.
It depends too on what failed and the symptoms. You mentioned the differential but that has nothing to do with the engine. The differential is part of the drive axle. Many engine problems can be repaired. It's not always necessary to replace the engine.
On the one hand there is really no manufacturer that is having common engine failures like that so two in one car is very unusual. On the other hand, even with a slow leak, you should have had some indication there was a developing problem if it was due to running out of oil. The engine would make a rattling sound and the oil pressure warning light would have turned on, and at that point there would have still been some oil in the engine so it would typically survive. If there was no warning of the immenent failure, that suggests a problem that was not oil-related.
To put things in perspective, I haven't drained the oil in my rusty trusty '88 Grand Caravan in over nine years, and it's a daily driver that I regularly pull a tandem axle enclosed trailer with that's bigger than the van! There is a very slow leak so I do add a quart about every 1200 miles and that keeps a fresh supply of additives in it. Please understand that what I'm doing is not neglect; it's abuse, and I'm not recommending anyone do that. I use it to show my students what some engines are capable of, (it has over 380,000 miles and has never been rebuilt). Sometimes when I ignore it long enough, it takes three quarts of oil to fill it, and it's only a 4.5 quart system. If my Dodge can hold up to that, I'm certain a Mercedez engine is at least as tough.
Any chance you can find out exactly what failed? Even if I can't tell you if it's due to running out of oil, I can suggest various things that could have happened and possibly how to narrow down the cause.
Friday, June 1st, 2012 AT 5:37 AM