The color of engine oil (i.e., internal combustion) may be used as an indicator of quality (see picture right). However, this may not hold true for your rough pump oil. I have heard on several occasions “oh… I don’t need to change my rough pump oil yet… it looks clean”. Welllll… “looks can be deceiving”! Take a gander at the various color grades of new and used rough pump oil I have amassed (picture below), as I was trying to find a correlation between oil color grade and my rough pump’s ultimate vacuum.
Despite the color discrepancies amongst these oils, all (except for the new) of these had degraded sufficiently such that my rough pump could not achieve ultimate vacuum for my specific application (50 mTorr). AND… color grade did not appear to correlate what-so-ever.
So… just because your oil does not “look too bad”, it may not be the best. This can be particularly important if your rough pump is working in unison with a turbo pump. I once lost power to my GC-MS and upon re-power my rough pump was not able to close the side-plate (check this out for more side plate discussions) because the oil was shot (someone… I do not know who would do such a thing… must have not changed the oil in a while). My system had probably been limping along like this for quite some time. Now the question was… how much undue strain was placed on my $6K turbo pump during this period!?!? Anyway, you get the point! Just like your car’s engine oil, you may want to consider changing it at regular intervals [e.g., the 6 months manufacturers’ recommended interval (i.e., under “normal” operating conditions)] instead of relying on the oil’s color.