Im sure you can get further replies on this subject. The 5-speed was indeed rare. I understand it was possible to find them in the 250SL and 280SL. I don't believe they were installed in the 230SL. These transmissions slow the engine down in the 20% range which is great on the highway. The W113's were geared too low, in order to provide quicker takeoff from a stop, and as a result the engines spin too fast at highway speeds. You will find some that will say the engines were designed to spin fast. Probably true, but not for long periods. I find that sedans which are taken care of properly with the M130 6 cylinder engines will commonly last 150 to 250K miles or even more. I also find that often SL's with the same engine will often need extensive engine work in the 80K to 100K miles range. I believe this is because they spin too fast for too long. Of course, there are variables here.
ZF did produce a batch of new 5-speeds in 2011 or 2012- I think about 250 of them. These could be ordered from a Mercedes Benz dealer or a Mercedes Benz Classic center in a kit form to install in the W113. The kit comes with most everythiing needed to install the ZF. You had to shorten the shifter rods and the front driveshaft. Initially these could be bought in the $9000 range. I believe there are still some available but they are now in the $17,000 to $19,000 range. This would be the most correct way to modify the car as it is a Mercedes factory supplied option. I did install one of these during 2012 in my 4-speed manual transmission 1971 280SL. It is really nice to put it into fifth gear and have the engine slow down and quiet down a lot. Much nicer than the constant high revs I had before. The 5-speed does not shift as smooth as the 4 speed. It feels stiffer during the shift, but not a problem.
There are other options to slow the engine that are a lot less expensive. The cleanest, easiest and least expensive modification is to install a rear axle from certain sedans which are in the 3.27 ratio range rather than the SL 3.9 or 4.0 or so range. That does affect the performance from a stop, but I think that most do not find this to be a major issue. There are also a couple of other 5 and 6-speed kits out there which can be installed also. I think there is a company in Texas that can even supply either a 5 or 6-speed along with the parts and pedals you need to convert the car from automatic to manual.
