GM-GMC: Engine Smoking, head gasket failure, coolant level

Question
My 1994 Chevy 3500 dully has a 6.5 turbo diesel engine and There's a light colored(grey/white) smoke comes out from the tail pipe when you frist start it, and it's less when the tuck warms up. But you still can see it when you're driving? Also the service light comes on when you reach speed of 55/60 mph and it turns off when you speed up or slow down. I don't know of any maintance on the truck since I just brought it not to long ago, but it was stop for a couple of months.  

Answer
Art,

White or light colored smoke is either unburned fuel or coolant getting into the cylinders of a diesel.

Your vehicle has OBD-I capability and you can go to http://www.kennedydiesel.com to learn how to jump the ADL connector to read the diagnostic codes.  Click on "Tech Tips" and then "6.2/6.5 Diesel."  You will see "Read Codes" on the right side.

The early 6.5 diesels had problems with injector pump failure.  GM extended the warranty to 11 years or 120,000 miles but your vehicle may be out of warranty.  You can read more at: http://www.dieselsuburbans.com/DieselSuburbanFacts.htm

Check the coolant level in the overflow bottle.  If you have to add coolant, you likely have a head gasket failure or cracked cylinder head.