Hello Ben,
The top of both intake and exhaust valve guides where the seals grab are smooth on a M127 and M129 heads. Later Mercedes W113 heads (M130)had exhaust valve guides with a groove cut in the top for valve seal to grab onto. The intake remained smooth on all W113 engines. After W113 production Mercedes heads had valve guides with grooves cut in boh the intake and exhaust valve guides for seals to grab onto. I beieve these grooves were added as time went on to help keep valve seals in place.
As materials improved Mercedes upgraded materials used in making valve seals on their newer cars. I believe the black rubber like material is Viton. These black seals are used in all the newer engines which have the grooved valve guides. The new valve seal kits for the older W113 cars inbclude the new improved black Viton exhaust seals which are designed to grab the groove in the top of the exhaust valve guides. Since the intake valve guide has no groove, as far as I know the white nylon type original intake valve seal is used (M130 engines).
Now since the 230-SL M127 engine has non grooved valve guides only the original style white nylon valve seals were supplied (as far as I know). Someone else may have more current information?
Next you must get to the root of the problem. None of these valve seal designs were defective. Things that cause valve seals to come loose are; wrong valve seals, improper installtion, worn valve (internal diameter) which causes valves to wobble around a bit, worn valve guide (outside diameter) which decreases seal gripping power. Extended periods of bad storage during which time valve seals can become hard and brittle or get stuck to a slightly rough valve stem may cause seals to jump.
The top outside of old valve guides where the valve seal grabs can become worn mirror smooth from seal movement, encouraging even new valve seals to "jump" (move off the guide). New valve guides have a micro ribbed machined finish which seals grab onto nicely. I have even seen later (280-SL) grooved exhaust valve guides with the grooves almost worn smooth.
Some tips; if you are reconditioning you may want to automatically replace all the guides. Machine shops only check the internal diameter of the valve guides. Bad wear on the exterior of old guides is often overlooked decreasing seal holding power. If you have a "jumped" seal,inspect the condition of the ouside of it's valve guide, replace the jumped seal with a clean dry new seal. As Dan and Cees have mentioned thouroughly clean the exterior of the valve guide before pressing it on. Oil may incourage the seal to jump again. Solvens such as "Brake Kleen" etc. work nicely for cleaning.
This information will be a little confusing to those who have not been through the process. Here is a photo of some of the different valve guide and seal combinations for the W113 engines;
Download Attachment: 
M130 engine valve guides and seals.JPG41.03 KB
Download Attachment: early type w113 valve seals and guides.JPG25.59 KB
The "Cylinder Head Discussion" post has some other information also.
http://sl113.org/forums/index.php?topic=1113,head,
Joe Alexander
Blacklick, Ohio
