Can synthetic and mineral oils be mixed?

And also, can oils from different manufacturers, viscosities, approvals and so on be mixed...

Generally speaking, mixing different oils in an engine, in any form, is a bad idea. It's bad mainly because it mixes together entirely different additives contained in these automotive oils, and there's simply no telling what chemical reaction such mixing might produce. It's absolutely obvious that the resulting blend can't meet any standards or requirements whatsoever. So you need to understand that mixing different oils of any kind is a last resort.

The question of mixing usually comes up either when switching from one engine oil to another (viscosity, type, manufacturer, etc.), or in cases where you need to top up the oil in the engine and don't have the right brand on hand. From the standpoint of your engine's longevity, I'd recommend thinking ten times over about whether switching in the first case is really worth it, and always keeping a litre of oil in the boot to avoid the second.

So just how harmful is this for the engine, really? A belief is common among car owners that when mixed, different automotive oils can supposedly curdle, especially when it comes to mixing synthetic and mineral oil. So, can they be mixed or not? The very existence of semi-synthetic-based engine oils proves that mineral and synthetic engine oils can be mixed.

That said, there are several very important caveats here:

1. Only base stocks from the same manufacturer are guaranteed to be compatible. Different manufacturers use different technologies to produce the base stock itself, and hence its chemical composition differs, so when switching from one type of oil to another (say, from synthetic to semi-synthetic, or to mineral), it's better to keep using oil from the same manufacturer for at least a few more service cycles.

2. Following on from the above, it's quite possible that, if you need to top up, it's safer to top up with a different oil from the same manufacturer (a different type, viscosity, etc.) than with an oil that matches on paper but comes from a different brand. Either way, though, you need to understand that topping up with a different oil means that you'll get that infernal mixture out of the engine at the very first opportunity!

3. When changing the oil, some of it (5-10% on average) inevitably stays in the engine and mixes with the freshly poured oil. So it's important to understand that the oil's properties change somewhat, which is why, for those same few service cycles, the oil should be changed more often than usual.

We also strongly recommend reading in more detail about how to properly switch from one type of oil to another.