You need to use the ESU smoke fluid with ESU locos, as some other fluids may have additional additives that may foul the ESU smoke generators.

I have used Suethe fluid in my ESU BR215, which worked OK, but now that I have the ESU fluid that's what I use.

It's the same for KM1 Gauge 1 locos, you have to use their smoke fluid for the same reason.

The ESU and KM1 locos have smoke generators that have better features than other manufacturers - Diesel locos that have smoking exhausts, steam locos that have cylinder steam as well as smoke. They've had these features for some time, but Marklin hasn't caught up H0 wise, and has only just started to in the last couple of years as far as 1 Gauge goes.

This is why the 'wheel has been reinvented'.

I come back to the point on 1 Gauge models with special features - cylinder smoke, etc. Marklin 02421 smoke fluid is "a large refill bottle with 250 ml / 8.45 oz. especially for 1 Gauge models with cylinder steam or for all smoke units."

https://www.maerklin.de/...s/details/article/02421/

i.e. Don't use 02420 smoke fluid in Gauge 1 models, use 02421.