Have just come across todays thread on crankcase pressure and perhaps I could add my 10 cents worth having just had the same problem. Even though I could blow down through the oil separator and get the oil filler cap to jiggle It was only after I replaced the separator completely with a new unit that I got satifactory reduction in crankcase pressure. While the separator was removed I also gently probe through the oil return pipe back into the pan with one of those flexible coiled wire drain uncloggers and found that it was somewhat constricted only allowing the separated oil to flow back into the pan slowly. Consequently on long runs the separator was slowly filling with oil and not draining quickly enough to effect adaquate pressure control. As to your query about blow by - have you ever experienced the dipstick being pushed out of its tube during the first few minutes of engine operation- if so then you might have excessive blow by due to bad rings. You might also try placing your thumb over the dipstick tube opening whilst forcibly holding down the oil filler cap to see what excess pressure is present - dont let the engine get too hot before you try this !! Both your car and mine are of similar vintage, and I take slight oil smoke from the dipstick tube and when the oil filler cap is removed as being par for the course on engines of their years.

Running on sythetic oil is, they tell me, better for cleaner PCV systems particularly on turbo engines, though it will really show up your oil leaks and possibly surprise you with more!!