1989 Mercedes 3005 rarely driven has slight blue smoke from exhaust.

What is your question?

Car runs great but when I just let it run in garage to keep battery up, engine will run normal then reave up and go back down. Was told possible need new valve stem seals. How much would that be and at this age I would like to keep car as I only drive 1,000 miles yearly and can't afford new car. Smog guy recommended I add "Engine Repair to new oil change. Any advice? How much $$ to fix Valve stem replace. What else could it be?Seems to need radiator fluid added more often than usual and hood is very hot after short trips. Am I looking at needing new engine? am I ? What cou