I have a 2009 4010 Trans Diesel Mule. I changed the engine oil and engine oil filter. I replaced the washer on the sump plug and torqued the sump plug to spec. after applying a small amount of lubricant to the sump plug and washer. There was nothing remarkable regarding this and I have completed this task many times.

When I warmed the engine, oil leaked at the sump plug.

I removed the sump plug and replaced it with a new OEM sump plug and new OEM washer. I cleaned and inspected the engine oil pan. I couldn’t see any issue. I lightly lubed the sump plug and washer and reinstalled it.

Oil leaked from the sump plug once the engine was warm. This was doing my head in.

I removed the sump plug again but this time replaced it with a “Smart O” sump plug purchased from eBay.

The Smart O plug fixed the oil leak. A friend uses a thin application of DOW 732 Clear Silicone when he replaces sump plugs. He swears by this product but was quite specific on only using the Clear 732 as this is best suited to this application. At the next service he cleans the sump plug and replaces the washer and applies another thin film of Dow 732.

Permatex Red RTV Silicone or Permatex Ultra Blue Silicone can also be used, but DOW 732 is most cost effective for him as he uses so much and purchases a 310g cartridge.

The Smart O that I used is an M16 -1.5.

Hope this might help others.