I replaced my series glow plugs with the parallel style a couple of winters ago. I got them from George_Murphy@compuserve.com. They cost the same as the serial ones. They fit right in to the existing large threads. No difficult rewiring necessary. I think Mercedes switched to parallels part way through the '79 model year. That's why the newer ones start better. With the parallel plugs in summer (cold engine) you have to glow for 4 sec. In winter, for 10 sec. In my case, they work fine down to -20C or -5F without preheating the engine. I can't think of any reason not to switch unless you want to keep your car original.

Gary