The car in question is not a Mercedes, but the concept is always something I think about regardless of make.

A few less mechanical people I know say to never do it. It can loosen up bits and cause it to go.

This car (Buick enclave) does have a serviceable tranny. I had it fully flushed at 100k miles. There is no removable filter.

20k miles later and it's shifting funky. Slipping a little. And the fluid leaks a little. Not much, just topping it off for the first time.

How do you all feel about this concept (I'm working on saying all instead of guys so we are more open to women on here. )?

Is this is an urban myth?

The shop said some Toyotas are non serviceable. I find this troubling.

20k seems really short to be a fluid issue...It sounds like something is actually wrong. As much as a fluid change might help, its probably not the solution. Do the obvious first and make sure its not low on fluid.

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

Hmmm. It was flushed at 100k. And now has 120k. And yes something is indeed wrong.