The oil separator (aka "breather box") allows the escaping crankcase vapors to slow so that the microscopic oil droplets can coalesce into larger drops. These largers oil drops drip back into the sump.
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Don Foster (near Cape Cod, MA)

Why do you need to reach the oil trap? The oil trap has no service interval: the only need to reach it is if it becomes plugged. This is rare.

I had to remove the oil trap on my '82 to replace the sump hose. It was a bit of a chore. Required removing the intake manifold. Could easily be done in a day, as I remember. Took me longer -- I was replacing the engine harness at the time.

Do yourself a favor: get the IPD flame trap relocation kit if your flame trap is hard to reach. A great, great improvement.
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-- Bruce / '82 244 (315k miles!), '86 745, '87 760Ti, '94 854

Oil Trap by name, oil trap by nature, it traps the soilds/liquids in the oil and allows just the hot air and vapor to breath into the airbox.

Engines need to be vented to allow gasses that build up to escape, but generaly with that gas you get some fluids/solids.

An alternative to a oil trap is a breather box, which is what I want to fit to my turbo, you basicly build a small box and fill it with steel wool or something and allow the oil breather to vent into that, it fills up sometimes with a little oil so you may need to drain it sometimes, but it's a better idea than letting oil vapor and gasses get sucked back through your engine.

Probally more info than you were after, but I was bored...