"The garage that distributes our lubricants purchases drums and sells the oil in 1-liter pails. If during an upcoming visit we wanted to rapidly confirm that the oil sold in the 1-liter pail is really our oil (meaning coming from the drum, since the mechanic could mix our oil with a cheaper one or could sell us another oil instead of ours), what can we do? How can we rapidly identify a lubricant? I think we might be able to use a digital refractometer (ASTM D1218) with a large scale to test the oil. What do you think?"

Unfortunately, there is not a quick way to verify that the oil your garage distributes is in fact your oil. To truly confirm it, you would need a full elemental analysis and the ability to compare the oil sample results to the certificate of analysis for each individual batch. This would allow you to validate the "fingerprint" of the finished oil and ensure that the additive concentrations match those of the original formulation.

Because a number of outside influences can affect the lubricant, it would be difficult to obtain conclusive results from a quick test conducted onsite. These variables would include how long the oil has been in storage, how much oxidation has occurred and whether the oil has been contaminated with water or moisture. Any of these factors could be enough to render "rapid" testing inconclusive.

However, there are a few tests that could provide some important clues about the oil, depending on how much mixing or substitution has occurred. These tests would involve using a viscometer to confirm the oil's viscosity or performing a base number field test to verify the oil's detergency. Keep in mind that if the oils are both of the same grade, the viscometer test will not be very helpful. In addition, the oil's base number can be affected by the length of time the oil has been in storage, so this test may not be the best indicator either.

While using a digital refractometer can offer an indication of differences in density, because of the many external influences that can affect the lubricant, this will not be nearly conclusive enough to determine whether the garage is mixing or substituting your oil. If you suspect this is happening, you may want to consider using a different garage for your oil.