If you check the FAQ, in the Crankcase Ventilation secion there is a lot of good info on this, and more info on methods for cleaning the oil trap. You can't get it open so you have to take it off and flush it through with some kind of solvent. Some folks have used kerosine or various engine oil flush products. I guess paint thinner might work also.
Neither of mine were badly blocked, so I just sprayed throttle body cleaner in the ports and let it dry out. Whatever you end up using, check a small spot on the oustide of the oil trap first to make sure it won't eat the plastic.
when you get it off there are two openings on the surface of the engine block underneath. The smaller (forward) opening has a plastic fitting inside it that holds an internal hose that feeds down to the oil pan. Be careful not to dislodge this hose as you will have to drop the oil pan to get it out. This is all in the FAQ, so it is well worth checking out. Clean carefully around the openings with a rag as this whole area tends to get real oily and dirty....
Dipstick o-ring is the little o-ring that is around the base of the "handle" of the dipstick and seals into the top of the dipstick tube. Its about 7mm inner dia x 2mm cross section I think. I just put a new one on. Big difference. If the old o-ring is hard or compressed it won't seal and air will get sucked in.
Good luck..
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Current: '95 945, 95k miles; '90 BMW 325i, 90k miles. Former Volvos, gone but not forgotten '85 245, 250k miles; '91 744T, 150k miles