Usually this is caused by a combination of two things... Poor drainage and too much supply, depending on the turbo. Some turbos don't seem to have this problem, probably because their oil passages are properly restricted internally. -3an seems to be the generally accepted feed line size for proper oil delivery. You can find restricted orifice oil adapters on ebay from a company called Function7 if it is still an issue. The seals aren't designed to positively seal all oil out, if the turbo can't drain the oil will find its way out through the seals, into the compressor and exhaust housings. My current setup doesn't seem to have this problem, I was a bit concerned my drain wasn't large enough, but its pretty near vertical so I think that helps. I recently took off my intake piping to mess with the carburetor, there wasn't any noticable oil residue in the inlet piping. On my turbo, looking into the oil inlet port it necks down to around a roughly .090" passage before going to the bearings, so I believe mine is internally regulated. I run a 3/16" fuel injection hose as my oil feed, simply because it was cheap and easy.
The turbo is a Precision 67GTQ. Compressor wheel is a 67mm inlet, I'm not sure of what trim it is. Housing is .75A/R. The exhaust is a Q trim wheel, housing A/R is .81. I'm not real sure when boost comes on, since I'm blowing through my transmission, so the turbo doesn't really have time to load up properly before the RPMs take off. I'd guess its around 3000rpm, maybe a little lower. It behaves a lot like a stock turbocharged car. Part throttle going up a hill the turbo starts to come in real nice. Not real surge-y, pretty smooth and well under control. If I'm cruising at 1/4 throttle and come to a hill, I can just roll up to 1/2 throttle for a second and back off to 1/4 again, the 1/2 throttle will get the turbo movin enough to keep it goin up the hill at 1/4 throttle. After the hill I just let off for a second and go back to 1/4 throttle and its back to normal operation. It seems like anything above 3/4 throttle is more of a boost control than throttle. At around 3/4rds throttle vacuum is around zero, so anything more than that is just adding boost.
Due to a wierd stumble right off idle, it tends to jump away from a stop, but that doesn't have anything to do with the turbo, just a carb problem. I have to see what I have for cash after christmas so I can get a wideband and start getting the tune reasonable.