White Smoke on 89 firebird

Hi, My bird has been smoking ever since I bought it. It usually smokes when it sits for a while and its a white smoke. I checked the engine oil and it was pretty dark, it also burns engine oil since the level was on add. I put new engine oil in (synthetic) and it looks like the more I put in it the more it smokes. I am not sure if this is a broken head gasket and the coolant is going into the cylinder, or bad valve seals. My mechanics told me that it could be a possible valve seal, but I want to make sure.
The car runs very well and I do not notice anything wrong with it except the smoke.
ALso, my compression runs between 120 and 130 (note a high altitude and 121000 mile engine).

Please help.

P.S. Is white smoke bad for my engine??

It doesnt smoke all the time. It only does it when I start it up. Now when I get more oil in there it tends to smoke while I drive it too.