“Relating to gearboxes on trucks, if the owner or driver doesn’t know if the gear lubes are synthetic, is there a fool-proof way to determine this without having to send a sample to the lab? Some oil manufacturers color their synthetic oils, while others don’t. What would happen if the oils were to be mixed or topped off with the wrong oil?”
The color of the lube is simply a dye. There are no standards, and manufacturers can and do change colors whenever they please. Unfortunately, there is no reliable way of differentiating between mineral and synthetic in the field. However, because synthetic base oils are white (meaning transparent) as compared to mineral oils, which have a darker natural color (due to aromatics, sulfur and other impurities), this may be a distinguishing factor. Note, however, that despite the fact that the base oil of a synthetic is white, the additives can add considerable color (darkening) to the finished oil.
In the laboratory, you could distinguish synthetics from mineral oil by looking at a combination of physical properties including VI, flash point, pour point and aniline point. There may also be different elemental additive chemistry.
Generally, in the type of application you are talking about, the synthetic gear oil will likely be polyalphaolefin (PAO) based. PAOs are very similar chemically to mineral oils, so mixing the two should not cause a compatibility problem, especially if both oils are the same API classification.
However, if a synthetic is required, such as for cold-temperature operation, using a mineral by mistake may cause other problems.
Also, be aware that in industrial applications, some synthetic gear oils are polyglycol (PAG) base stocks, which are chemically incompatible with both PAO synthetics and mineral oils. In this case, mixing will result in serious incompatibility issues.