As professional engineers of 50Hz to 60Hz frequency converter (see on http://www.frequencyconverter.net), we should not encourage unsafe cheap options as it may turn out to be penny wise pounds fool. Under the given circumstances, you may share the details of insulation and other materials proposed to be used in your 50Hz to 60Hz frequency converter, and the environments of its placement with the frequency converter manufacturer, who may be in a position to provide oil best suited for your frequency converter. One one hand, this 'infantile' failure may be solved by a review your manufacturing process/details. On the other hand, review your design and component selection criteria for an optical sensor and choose one best suited for your 50Hz to 60Hz application.You're fighting city hall on this one. If the 60Hz 50Hz frequency converter manufacturer won't give you his blessing then you're stuck, you're going to have to qualify that piece yourself if it must be in constant contact with the oil. On the other hand if it's part of an assembly that's indifferent to the environment except that has to be cooled by it, then why not encapsulate it in a medium that is known to be unaffected by the oil, or simply put the subassembly in a hermetically sealed container designed to the frequency converter 60Hz to 50Hz.
