I have cleaned them. This involves disassembly, not dumping some magic chemical into the oil.
Robertson #3 bit, in your favorite brand of hex-bit socket. The plugs have a 3/16 female square wrenching surface, but typical 3/16 square pipe-plug sockets are either too tapered and with rounded-off corners on the male square (Snap-On) or just plain too weak (Mac). You'll make a mess of the plug or break the socket. Which is exactly why no-one removes those plugs.
My Mac man helped me, nearly twenty years ago; the #3 Robertson bits are 3/16, or close enough, HARD and NOT TAPERED and with adequate corners. They fit the plugs great. Of course, you'd want a bit-socket to hold the Robertson hex shank. My Mac socket, and USA Craftsman sockets are decades old, and long discontinued. But there's plenty of other choices.
Photo 1. Original plugs removed, brass hex-head plugs ready for installation. Oil gallery brush ready to clean oil passage. Smaller brushes needed to clean transverse passages to main bearings.
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I cut the Mac bit shorter by removing the end of the hex shank. Optional--you wouldn't have to, and other brands of Robertson bits may not be made the same.
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