This is a verrry common problem, and the fix is a $2 seal (and a little grunt work) for the top of the oil return line. Oil also oozes from the shaft seals, so you have a little oil in the duct running from the turbo to the intercooler, and some carbonized oil inside the exhaust line as well - this is normal so don't let a mechanic tell you to replace the turbo.

The first sign of a failing turbo is oily smoke under hard throttle, caused by oil escaping the rear shaft oil seal and getting burned up in the exhaust system. Turbo failure is rare in the FWD Volvos, but make sure you do regular oil changes. Switch to synthetic for better flow characteristics in the turbo.
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David \\ (98 S70 T5SE Black, misc mods (mostly lighting), red calipers) (92 940GLE)