My 1977 300D has approx 193,000 on it, a relative baby. My dad was religious with oil changes, but only adjusted the valves 'a few times".

I didn't notice any of the negative symptoms, but I did have the valves adjusted after I had put 5-6,000 miles on the car. I did notice a reduction in noise, quicker starts, better pick-up. I also changed fuel filters just after I had the valves adjusted and between the valve adjustment and the fuel filter change out, performance really improved, pick-up/top speed. (for a non-turbo, "really improved" means from very slow pick-up to slow pick-up and top speed from 75 to at least 85 mph.

My plan is to have it done annually by a tech, and have him check out the timing chain (original) as well. I do DIY for most stuff, but would rather have a tech tackle these issues. For $120, it's a bargain.

I'd say if you don't know the history, have it done or do it yourself. It's not a big price tag, even if you have a tech do it and you'll notice the difference.

JCD