Analysis:
At present, there are many brands and varieties of oil-immersed capacitors outside, including round ones and vertical ones (just like vertical electrolytic capacitors). Some are square, which may be the kind you said. But what you should pay attention to is whether it is really soaked in oil. Some oil-immersed capacitors will say paper capacitors, but the capacitors are completely sealed with a metal shell, and there is an obvious seal on the shell (generally it feels like a tin seal bump). In other words, this capacitor is an oil-immersed paper dielectric capacitor, and some of them write "paper in oil", which is more obvious. In general, square green or unpainted (that is, those made directly of aluminum shell) are mostly oil-immersed, as long as there is a sealing opening on the shell (generally at the bottom, next to the terminal or under the side). This is the problem to be considered for cylindrical small capacitors. If you don't indicate that it is oil-immersed, it can be proved that it is oil-immersed, because people didn't think that oil-immersed capacitors were such good things as they are now, so sometimes they don't indicate that they are oil-immersed. Oil-immersed capacitors are characterized by small capacity and high withstand voltage. Generally, the withstand voltage is above 200 volts, and some can be as high as 1500 volts. But the capacity is very small. Generally, a piece of capacitance as big as soap will not exceed 8u. Oil-immersed capacitors with very small capacity, such as 0.0 1u and other second-hand capacitors, are mostly bad, so it is not easy to find small capacitance values. On the contrary, I have never encountered a bad oil-immersed capacitor above 0.25u, and the insulating oil of the oil-immersed capacitor is different from that of the oil-immersed transformer. If the insulating oil of oil-immersed transformer is not satisfied, you can hear the sound of "rushing" when you shake it, but the capacitor can't hear it. I have removed the oil-immersed capacitor, and the oil in it is very thick and toxic (some capacitors will be marked with danger on the outside). In large oil-immersed capacitors, the AC voltage is sometimes marked. It doesn't matter, it can still be used. If the AC voltage is marked with 300V, the DC voltage will be higher (e.g. filtering). There are many brands of oil-immersed capacitors on the market at present. Personally, I think the quality of TCC and Dubilier in Britain and Sprague in America is good. There are many American brands of GE capacitors, and the price is relatively cheap. The capacitance quality of Sprague is better than that of GE, so it would be better if you could buy Black Widow or VitaminQ in Sprague. In addition, there are some second-hand oil-immersed capacitors, such as ERO and RIFA, but it is not easy to see large-capacity ones. It is often seen that there are long barrel capacitors selling GE, which are printed directly without painting. GE's trademark is a round flower logo, which looks like oil immersion, and its mouth is sealed, but its weight is extremely light, so it has not been bought for trial use. Domestic oil-immersed capacitors have a huge stock in China. We have never used domestic oil capacitors, so we dare not evaluate their quality. Japan's oil-immersed capacitor, NEC brand, once used 0.47u as the frequency dividing capacitor, and the sound was too slow, which seemed a bit "oily". In Guangzhou, there are many high-quality oil-immersed capacitors, square oil-immersed capacitors produced by WE in the early days, and some very, very old vertical cylindrical capacitors (they look a bit like electrolytic capacitors, with unpainted aluminum shells, which can withstand more than 800V, and the capacitance values are generally 6u and 4u, with a diameter of about 30mm and a height of 150mm), sealed, and most of them are Dubilier. I believe that these things were once designed for civil use, so their appearance is different from that of industrial squares, but now they are rare and more expensive than squares. There is no way to compare which capacitance is better with ordinary instruments, only its capacitance, Q value and so on can be measured. Among the oil capacitors in my hand, TCC, Dubilier and VitaminQ are better, with a Q value of about 350, VitaminQ close to 400, and GE's is worse, with a value of about 240 to 270. There are some other brands of oil capacitors, which have not been tested because they have little confidence in them. The capacitance of Sprague without VitaminQ is worse than VitaminQ, but it is equivalent to the Q value of TCC. The above is my experience in playing with oil capacitors.