1. Serious Blow-by - out into the engine or into the intake manifold through the PCV Valve
2. Bad Valve Seals - Usually smokes alot during hard acceration and sometimes at idle
3. Bad Seals in turbo - Do as poster said above and check for oil
4. Oil Leak - Pretty obvious
5. Bad Headgasket - Mixes with coolant

Only things I can think of off hand.

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Jerrome B.
1995 GSX - Race
1997 GST - Drive
1995 GST - Who the hell knows