Older 300SD's will eventually spring a slow leak in the system. It's amazing that they do work without any problems for 15-20 years. The A/C in my 81 SD was "rebuilt" a month ago at the dealer's recommendation after they discovered a leak, and they will not just recharge with R-12.
After various valves, gaskets, and new evaporator? (the small cylinder thingamajig mounted on the wheel well) and new oil (?) plus recharge of R-12 brought the system back to new condition. Well worth the $500 they charged - Although I love tinkering, did not want to mess with it and risk not doing a thorough job.
The dealer mentioned in most cars they just refurbish and refill with R-12.
If the compressor is shot, then they recommend converting to R-134.