Earlier there was a thread on valve oil and we got sort of "goofy" and strayed a bit. So I contacted my son, who is a Chemical Engineering student (senior) at Purdue University and posed the question regarding the different types of valve oil and which is the best. As we all know, there are two main types of valves: piston and rotor. As such, there are valve oils to accomodate each type of valve. The difference between the types of valve oil is primarily the viscosity or weight. In the other thread, I stated that valve oil to many brass players is a personal thing and claim one brand to be better than others. Valve oil is basically a petroleum distillate that have properties similar to kerosene, as many of you are most likely aware. This is the most commonly used type of valve oil, ie., Al Cass "Fast" valve oil. For rotory instruments, the same composition is used only it is a heavier weight, such as Holton's Rotory Valve Oil. Rotory valve oil is also excellent for older piston instruments. Synthetic valve oils are now becoming more popular and being used more and more. The characteristics of synthetic valve oil include greater compatibility with other valve oils without the "sticking". Many types of the "mineral based oils" reacted with one another forming a thick residue that many times required the instrument to be taken apart and a complete cleaning of the casing and valves to restore satisfactory operation. Synthetic valve oil has a slow or even in some cases a total lack of evaporation that results in fewer oiling to keep the valves functional and less dissipation on contact with moisture inside the valve casing. Some synthetic valve oils, such at Ultra Pure, have an additional advantage because they do not act as thinners if they came into contact with slide greases if the oil was inserted into the valve through the tuning slide and fouling the valve. My son stated that most of the trumpet players at Purdue used a synthetic valve oil.