Too-low oil viscosity

The exact opposite situation arises when an oil's viscosity is lower than the norm. Nearly every car-oil manufacturer today makes so-called fuel-saving oils, with reduced high-temperature viscosity. This specifically concerns viscosity at high temperature and shear rate, HTHS (above 100 °C), so these oils have the same SAE viscosity index as regular ones. What sets them apart from regular oils is their quality class and the carmaker approvals they carry. Specifically, low-viscosity oils correspond to the ACEA A1/B1 and ACEA A5/B5 quality classes.

The problem is that special engines are built for oils like this! And using such an oil in a regular engine not designed for such low viscosity is simply dangerous. The point is that at high temperatures and high revs, the film that forms on the friction surfaces becomes too thin, which cuts lubrication efficiency and sharply increases oil consumption through burning. Under the wrong combination of circumstances, the engine can even seize up.

So, running an oil at a lower viscosity than the carmaker requires is far more dangerous than running it too high. That's why you should never use ACEA A1/B1 and ACEA A5/B5 engine oils, or any special oil that lists only a single manufacturer approval, unless those quality classes or approvals are actually listed in your service book or owner's manual.