I bought into the idea of "lifetime transmission fluid" by Mercedes. I have 146,500 on the car and have thought about changing the filter in the transmission. No shaking or mis-shifting at this time. Should I replace the filter then reuse the old fluid, replace the old fluid or do a flush?
Clarification: What is the model year? Adding gearbox expert @Dan to the discussion.
Don't do a flush, that would be almost certain transmission death.
What does the fluid look like? Does it look just a little dark? Or is it burned and gritty? If the latter the grit is from friction material suspended in the fluid that's worn off of clutches and bands. In such cases it is frequently the friction from that grit that is all that's keeping the trans working.
If the fluid is just a bit dark but not burnt and gritty what I would probably do is perform a drain and fill, which is a partial fluid change. Drive it around for a while to let the new fluid mix in. Then drain and fill a couple of more times (driving around between times) after which you'd have about 90% fresh fluid. This will introduce new fluid gradually. If there is a filter, change that as well with the last drain and fill.
HOWEVER - this is only generic advice since I am not experienced with Mercedes. You should check with a Mercedes mechanic to see what is best for that specific transmission based on their experience.
Yep, if the fluid is only a bit dark you can replace it.
Knowing Mercedes 5G-tronic transmissions, if the fluid is dark it usually will begin to have torque converter lockup (this generally tends to happen at 150k km even if you do replace the oil) and other solenoid issues, so knowing it doesn’t seem have the symptoms typically associated with those I think that you should consider inspecting the fluid and replacing it if it’s not completely dark.
(Maybe on this specific Mercedes I think you should replace the fluid even a couple of times with short 5-10k km intervals to wash out the friction pack dust that is usually the number one cause for these to go bad)
(Also I’m assuming you have 150k kilometers and not 150k miles. without a fluid change is pretty much impossible almost on any modern automatic, even the best ones start to stutter at around 125k miles if you don’t keep the fluid clean)