Car longevity is not a spectator sport. You can't just sit back and watch and expect your car to last forever. There are times when you'll have to roll up your sleeves, get out some tools, and get in the game. The easiest farm team league is to replace engine filters at their correct service intervals.
The engine air filter for the Mercedes 300SL is huge, and priced accordingly. The bill for changing it can easily be brought down to size with a little effort and a little time. We used a MANN C40-193 that we got from a small auto parts store. This is a popular enough filter that any chain auto parts store should have it in stock or be able to get it for you within a few days. You can always go to the parts department at the Mercedes-Benz dealership, although this is likely to be the highest priced alternative
The major savings comes from the clock. The hourly rate for most repair shops is over $75 an hour and you'll have to make an appointment and disrupt your own schedule to meet theirs. Replacing your own air filter will take ten minutes of your time and can be done when it is most convenient for you. Even if you have never done car maintenance before and have no tools, you can get a set of screwdrivers and metric wrenches for far less than the hourly rate a repair shop would charge for just this job. Don't forget, the tools will then be available for you to use on the next task and will continue to cut your cost of ownership.