Samp;G Tood Aid company invented this type of oiler and patented it (US3807530) in 1974. This model is the 98700 with 3/8quot; ports however I own the 98800 which is exactly 100 identical except that it has 1/4quot; ports. I also have the smaller 98500 and find it's of similar design and quality although in my opinion slightly harder to fill. These oilers work great and do not leak at all when the gaskets are in place. Many cheap knock-offs are available on the market but some of them are very poorly machined and most of them use a single-hole orifice for oil metering. The 98500 and 98800 are beautifully machined so they cannot leak and they contain a true micro-porous filter in place of an orifice for metering so they cannot clog. There ARE indeed other good units available on the market but the 98800 is by far the Cadillac of sight-glass type in-line oilers at an exceptionally good price. The Samp;G Tool Aid units additionally have very sensible spacing between their internal metering drum and the plastic sight glass; this is absolutely vital to operation because wetting of the metering brass hole is maintained by the surface tension and viscosity characteristic of the oil. If the gap is too large due either to faulty design or loose tolerances then the oil will NOT travel down into the metering hole. I had to discard a cheap Chinese unit for this reason alone. There is one important think to keep in mind when using this type of oiler regardless of manufacturer and this is that the amount of oil metered out is inversely proportional to reservoir fill level and directly proportional to air-line pressure drops. This means that when the reservoir is 100 full then no oil will be metered out and when the reservoir is almost empty then lots of oil will be metered out every time the airline pressure drops. Therefore always try to keep it at least 1/3 full but less than 4/5 full. Also, when you are finished with your tool, hang it such that the female-threaded side of the oiler points up and the male-threaded side points down, otherwise a large spurt of oil will inject when you depressurize your line. IMHO these oilers work fantastically well and so there is no need to contaminate your long hoses with oil. My motivation for sharing of this info is solely charitable and I have no affiliation with the manufacturer.
Rating: [5 of 5 Stars!]