The brushes are porous by nature, anything carbon-based is porous (heard of activated carbon? It's REALLY porous!) They are an alloy of carbon and graphite, sometimes with metal mixed in (copper or lead or both) depending on what the application is. The carbon provides the conduction, the graphite is the lubricant. Oil will weep up in the brush and sit in the "pores" of the brush, then sweat out every time it gets hot (which is when it runs).

The commutator is also not pure copper, it is a brass or bronze alloy with a very high copper content. Some are lower in copper and actually have a yellow color to them. The alloy has a much harder surface and better wear characteristics compared to pure copper. Pure copper on a commutator would wear down very quickly in a motor designed for constant use such as a fan. Unfortunately, the brass and bronze are also slightly porous and will soak up oil like a sponge. It doesn't take much to cause nightmarish problems later on.

If you have the ability, it would be a good idea to clean the comm and brushes to get the oil off. If you don't care, that's one thing, but time is a lot cheaper than a replacement motor. Just sayin'...

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]