One of the things with synthetic brake fluid is that it won't absorb water which seems like a good idea ( and it is ) but any moisture can lay in lower areas of the braking system and start to rust the lines from the inside. On the other hand it won't cause brake cylinders to rust which could be a better option on the 230SL.
It's a real problem for cars like 190SL's where the vacuum connection is right at the bottom of the brake booster. Brake fluid doesn't burn all that well if it gets into your engine and will produce a white smoke that you can see after you apply the brakes. Synthetic produces less smoke and you might not see the problem until your engine is filled with a nasty mess. It will ruin an engine - in fact they both will only DOT 4 will give the white smoke warning.
Our cars have the vacuum connection very high on the booster so there's very little risk of brake fluid getting into the engine. Almost every booster I look into has some amount of brake fuid inside due to leaking master cylinder seals ( which is why I'm looking to begin with ). It doesn't seem to affect to booster all that much though. I draw out as much as possible during master replacement or I pull the whole booster and drain it out and let it dry.
