That really doesn't sound like a problem (oil in the tube).
You are on the right track replacing the plastic tubing parts. It took me a while to track down a crankcase air leak on my 98 (rough idle). Turned out there was a crack in the dipstick tube that only showed itself by leaking a few drops of oil after a track day. Replaced that then found out my AOS was bad. When replacing the AOS the original tube to the throttle body shattered while removing it.
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