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Hi. What do you mean by base? Do you mean the oil pan?

If this has a fuel pump that is lubricated by the engine oil, it may have a bad seal allowing pressurized fuel past that seal and into the engine oil.

yes.

DMiller said:

Hi. What do you mean by base? Do you mean the oil pan?

Click to expand...

Typically, the primary source would be the high-pressure fuel pump, they are normally gear driven and lubricated by the engine oil, so if they leak internally, fuel will get into the oil.

Other sources can be:
-leaking injectors, when the tractor sits, some fuel leaks out into the cylinder, and the fuel runs past the rings.
-mechanical fuel pump (low pressure pump to supply the main high pressure fuel pump), it also will be lubricated by the engine oil, and if it fails, fuel can get into the oil