my rb25 was doing that too, i only just made it home in an oily mess, pulled the engine down and had melted ringlands on a piston. everytime i got on boost it would pressurise the sump and pop the dipstick out. it ran fine other wise.
i would recomend getting a comp test asap, will show up any difference in cylinder pressures, may not be a big difference depending on how bad the damage is.