Again, I was thinking mainly of Engine oils, and that a Comma 5w30 will mix happily with Quantum 5w30 and a dash of Halfords 5w30 won’t do any harm. I should have made that clear, that part of my post was sloppily worded and should be disregarded. If all oils mixed freely and did a good job of it, well, we would sell one product and I’d be out of a job.

Machine lubrication is another pot of fish. Gear boxes in particular. It sounds possible that your mistake was mixing 2 incompatible gear oils, if it needed mineral gear oils or PAO synthetic gear oils (which are miscible), and you used a PG synthetic type (or vice versa) it won’t mix and would cause severe problems. Other things to be mindful of are oils containing Zinc if silver is present, and oils containing Sulphur/Phosphorous if yellow or white metal is present. Compressors are another minefield for the unwary, as is selecting greases.

I’d always recommend checking with the manual before filling, and asking your supplier!