That mixing light microscope immersion oils from different vendors can be a bad idea? 

Be careful when reviewing your samples at high magnification on two different brands of light microscope. If you find that you need immersion oil to see fine details in your sample, please do not “mix” two different brands of immersion oil. Vendors tell us that they have seen instances where the two different synthetic oils combine on the lens surface and form a sticky substance that’s very hard to clean off the lens.

If you find you need to use two different light microscopes, try to first clean the original microscope immersion oil off the slide using lens cleaner (such as Sparkle*), followed by a wash with a gentle stream of 100% ethanol. Dry with lens paper to avoid scratching the coverslip.

* Sparkle is a commercial product often recommended as appropriate for cleaning microscope optics. The "optical grade" does not include the purple dye.