They tow it to the dealership and demand that it be covered under warranty, obviously.
In reality, nothing. Usually. Lubricating oils are good at lubricating and coolant is a good heat transfer medium. Most often the fluids are chosen for a specific effect under specific circumstances. Lots of factories changed to 20W engine oil from 30W to gain a half a smidgen more efficiency. You can use either without breaking the engine. But if you have a limited slip differential, you're going to hear the clutches chattering if you don't add the friction modifier fluid. I'm not an engineer, but I don't like hearing noises like that. Coolant flows through several pieces of metal. Aside from heat absorption, there could be issues with electrolysis or corrosion. I don't know, I'm not a metallurgist. I don't care, either. What does it matter to me what can be substituted when I have the factory specified fluids on hand?
When we get an oddball European car coming through, that gets really annoying. I'll waste a lot of time making sure the oil we use meets the OE spec. Hell, just for our own vehicles we've got two different kinds of 5W30 synthetic, in addition to 10W30 synthetic. We also have 0W20 and 5W20, 0, 5 and 15W40 synthetic, all for our own vehicles. There's another kind I don't remember the description of because we've never sold it, but I got a case last fall because I came across something that called for it. And it's different somehow.
I don't know or care about the whats or the whys. I get pissed at people (not customers) who pretend to care, but really don't. They just want
The Dealership to tell them that the cheapest fluid at Wally World will be exactly what their car needs- so if their neglectmobile blows up a week later, it's somehow the factory's fault and they can yell about getting a new engine for free.