Do NOT use automotive brake parts cleaning solvent. This type of solvent will attack some types of insulation and insulating varnish. The motor is classified as "lower voltage" (at least around the powerplants I worked in over my career). As such, using the soap solution suggested by Rons, above, is a good way to go. Do not get too aggressive with brushes or scrapers or poking around with sticks to get the congealed sludge out of the windings. Old motor windings were insulated with cambric (a fabric) as well as "fish paper" between the layers of windings. The windings were then 'laced' or tied tightly with either twine or cambric tape, and the whole assembly was usually dunked into a vat of heated insulating varnish, possibly autoclaved, then baked. Over the course of many years, these insulating materials can become brittle or break down due to prolonged exposure to oil. It does not take much to 'open' a leakage path to ground in old motor winding insulation, and oil sludge makes a fairly good conductor. The soap-and-water method and care to avoid poking and aggressive scraping is about all I'd recommend as well. Once the washing is done, rinse with clean water, no soap, no pressure washer. Use a garden hose nozzle on fine spray.
After the washing and rinsing, I'd use a heat gun at some little distance (to avoid putting direct blasts of higher temperature air on the windings and insulation). Up-end the motor housing/stator with the end bells removed to let things drip dry. A little help in the form of a "tent" with something like an electric space heater (aka "Milk House Heater) , or even hanging a bare 100 watt incandescent bulb inside the windings to throw some heat would work well.
Be careful about using compressed air to blow the windings dry. "Shop air" often has condensation and tramp oil from the compressor along for the ride.
Once you have the windings cleaned and dried, inspect them for any visible damage to 'lacings' or insulating materials, as well as to the insulation jacketing the wiring leads to the 'peckerhead' (the term for where the external wiring connections are made on a motor or generator). You may want to replace the insulating sleeving on these wire leads as old insulation is often quite brittle and cracked. Modern practice is to use a woven 'Kevlar' sleeving (McMaster should have this). Touching up any suspected damage to the old insulating materials and insulating varnish with 'Glyptal' is also a good idea. Glyptal is an insulating varnish that has been around since Noah's chief engineer used on the windings of pump motors aboard the Ark. It is usually a red varnish, put on mainly by brush, and has high dielectric strength and makes a good repair to insulating materials where you think you might have some cracking/'leaks". Two coats of Glyptal can't hurt if you see something suspect.
I am reminded of an incident that happened some years ago at one of our hydroelectric plants. Suffice it to say one guy, doing routine maintenance on the station drainage and unit unwatering pumps got a bit befuddled. Following 'preventative maintenance', the work order called for testing the pumps by partially opening an 8" butterfly valve which was normally used for draining the draft tube and turbine casing of each unit. The PM called for flooding the station sump to fill it, then checking how the high sump level alarm came in, how the pumps came on line, and timing how fast the pumps pulled the water down in the station sump pit. I was home, asleep, on cold January night about midnight when I got a call from the mechanical maintenance supervisor. He reported that water 12 feet deep flooded the lower elevation of that powerplant (he was also at home and the control room had dispatched an operator to that plant, which normally ran remotely at night). OK, sez I, if ----- was doing the PM on the station drainage pumps, he likely left that 8" butterfly valve (worked by handwheel and 'stand', with the valve down in the sump pit) partially opened. Maybe -------- turned the handwheel the wrong way. From the sounds of it, the lower elevations of that plant are 'tidal' (meaning holding the same level as the river on the tailrace side of the plant). The maintenance super asked me what to do. I replied: have security call out electricians, have them bring a stake rack truck with 480 volt power cable for immersion in water, wiring supplies, and be prepared to rack out the 480 volt breakers for the station drainage pumps.
Call ----- the construction equipment rental shop, they have a 24 hour hotline. Have them bring out some large submersible motor driven pumps. We could not use diesel driven pumps due to location inside the plant, nor could diesel pumps located outdoors take that kind of suction lift. "How Many Pumps should I ask for ?" followed. "Start with a couple of 10 inchers and be prepared to keep adding pumps until the pumps get ahead of the inflow... It's anyone's guess how far open that 8" valve is, and who knows how many GPM the inflow is..." I arrived at that plant sometime in the wee hours with the mechanical super. I joked that we may as well be trying to keep up with the hole in the Titanic as more pumps were temporarily wired in and lowered down using the plant bridge crane. I remember looking down into the water in the lower elevations and seeing the 'vapor tight' lights still lit. SOmetime the next day, enough pumps were on the line to get ahead of the opened 8" valve. In nothing flat, the chief electrician was having his crew reconnect the breakers on the two station unwatering pump motors, up on the main floor at the Motor Control Center. I asked the chief if he wanted to megger the motors before restarting them. His answer was: "Low voltage motors, fresh water with some turbine oil, they'll be fine." He was right.
Later on, the crews opened those motors and really washed and cleaned the windings as the raw river water had some silt, and there was a film of turbine oil and light debris inside those motors. But, they started just fine and pulled load prior to being re-cleaned. Moral of the story: wash the winding with soap and water, dry them out and your motor will be fine.
As for the guy who got befuddled about that valve, he was an old friend who often had my back, so we covered for him. Corporate was a pack of dummies, and it was easy enough to cover for him. We all got commendations in our records for dealing with the flood, even the guy who'd left that valve open. Some years later, I stood up in church and gave an 'off the shoulder' eulogy for my old friend, speaking up for him one last time.
