Showza, had a very similair prob to you. when i did my rb30 i bought a vg30 and had it checked out by the same shop in question, replaced the rear seal as he said it was in need of a change. As soon as it was installed it made a slight whine on boost, didn't really take much notice as it wasn't really noticeable and didn't think it was abnormal! Turned out the rear seal was screwed from the start, and after only 1000kms i found oil in my cooler and piping!