Some off market replacement parts are fine. But, for example, on eBay you'll find a dirt cheap O2 sensor. You get what you pay for.
As for mixing chemicals, just keep in mind that when you mix two different chemicals, you end up with a third chemical.
And I can't believe MB has special head gasgets made for this thing that just happens to need this "special coolant"
Ah, but they did. So did other manufacturers. GM went with Dex-cool (a Texaco product) which is a non-silicated Organic Acid Technology (OAT) coolant, and they engineered the cooling system and all the parts, including the seals, that come into contact with it accodingly. Chrysler, Ford (and most others) use a (slighltly) silicated Non Organic Acid Technology coolant (HOAT). Everything was engineered accordingly. The old school green stuff is highly silicated, organic technology, which gives good protection, but breaks down very quickly (30,000 miles or so) and when it breaks down the silicates turn to pure evil.
But it's more than just silicates being the difference between OAT and HOAT coolant. They are both glycol based, but they are different chemistries.
Also, the G05 HOAT coolant has between 250-325 ppm silicates. Again, Dex-cool has zero. The one-size-fits-all coolant that's good for all amuminum engines, how many ppm does it contain? I dunno, but I'd sure wanna know before I put some in my radiator. Does it contain none, which will dilute the G05 that's already in there? And what chemistry is it? Is it miscilbe, or will it create a new chemical when mixed?
Daimler specifically recommends against mixing G05 (and G48) with any other coolant. GM specifically recommends against mixing Dex-cool with any other coolant. A one-size-fits-all coolant can't possibly be good for G05 and Dex-cool at the same time. No way. Basic chemistry tells you that, and the unknown factor of the silicate level makes it iffy at best.
In any case, he's seen the wrong fluid cause problems. With ATF, the wrong fluid will show itself very, very quickly. The wrong gear oil for the rear end will, too. Same with power steering fluid. Wrong brake fluid will show itself in a matter of weeks or months. The wrong coolant will show itself, too, just not as quickly, and it might not show up directly as a coolant problem, as it might be a coolant-related part that goes bad, and the connection might not be evident. But if all the bodily fluids other than coolant cause problems, it stands to reason that the wrong coolant will cause problems, too, even if you are completely unaware of silicates and chemistries and OATs and HOATs and stuff.
There's a few things I know, and a lot that I don't know, but one thing I know for sure, if you always stick to the list, the list won't come back to haunt you. They can't point their finger at you and say, "You used the wrong stuff!"
