if its not run up to propper temp the pistons wont expand to there running tolerance there smaller when cold as so to say. there desighned to burn oil from new i think its 750 mltrs per 1000 miles. what you might of seen on top of the pistons was probably more prtrol mixed with oil. the cold start on these want be liberal pre cat days and all that. so add on 30 years a bit of wear is it servicable thats the question. when its warmed up propper is it blue smoking bad when you give it the gun or is it a slight puff of blue haze momentaraly. on over run on a steep hill foot of the gas is it chucking out blue smoke if not its servicable. mine will see of plugs the oil looses its viscosity after a couple of winters just running it up with condensasion cold start fuel