This is from Turtle in a PM hope he doesn't mind...save him some typing...
I do know that, generally speaking, the oil separator is usually a small plastic (usually) box below, or by, or inline with the intake manifold. Meaning, just upstream and below the air intake. I need to pop the hood and scope mine out, I guess. It's part of the crankcase ventilation system.
As the name sort of implies, it grabs oil and blowby gases from the crankcase, internally separates the liquid oil from the gases, then routes the oil back to the crankcase while the "purified gases" get routed upstream to the intake to be burned. It's the same thing as a PCV Valve, except that a PCV valve just reroutes everything back into the engine to be burned, whereas the Oil Separator separates out the oil from the gasses, then sends the gasses back to be bunred and sends the oil back into the crankcase.
I know that if you don't change the oil and oil filter often enough, and the engine air filter, or if you use a non-approved engine oil, a clogged Oil Separator is almost certainly gonna be the result. But, because of how it works, I gotta figure that no matter how often you change the oil, it's eventually gonna get clogged and have to be replaced. But if the oil isn't changed often enough, and it gets thick and sludgy, that'll do it, for sure.
A clogged PCV Valve ends up making an oily mess, excess smoke, fouled spark plugs, valve cover gasket or main oil seal leakage, etc. This is my first diesel, so other than the obvious similarities, I'm simply not familiar enough with it to even fake being an authority on it. hehe Far as I know, diesels don't have spark plugs, and the like, so I'm not sure what all happens when an oil separator gets clogged. Also, a PCV Valve generally gets replaced every 40,000 miles or so, and it's cheap, but I think the Oil Separator for a Sprinter is about $225.
But, I don't see an Oil Separator listed at any of the places for routine maintenance items (like Europarts of San Deigo, for example), so I have to assume that it's not an often replaced part, as in a 40,000 mile PCV Valve.
Price on invoice is $328.48 for separator plus labour of course....