My L2800 is 6 years old and does not have the "safety filter" inside. Do you have a part number for it?
My current filter (and the type that came new with the tractor) is a single cartridge type filter Kubota #TA040-93230.
If everything is working correctly how do you get oil or other containments on the filter in the first place? Seems like a bigger issue if you do.
I have never washed a paper filter I just tap out the dust every few months and replace each year... You should avoid using compressed air as well unless it is super low pressure and from inside out. A new filter is Cheap insurance considering the damage dust can do.
I do replace the inner filter once every few years as well.