That depends. Oiling the prefilter obviously helps it trap more dirt thus extending the life of the paper filter. SO if you look after your own maintenance and service your prefilter a couple of times during the season- oiling it properly won't hurt. What I do for a wrap style filter that I am sure sees the light of day once a year is spray it w/ a light filter oil. That is unless there is a service directive not to do so. Some say use WD 40 as that just leaves a light trace as well.

If the preCleaner/filter is Foam. Then yes it needs to be cleaned and oiled lightly as needed. Use a good foam filter oil.

Precleaners/filters are cheap compared to the air filters.

Hmmm I would go with the Dealers recommendation. If the Filter is a pper element no oil should be applied . If it is foam then Filter Oil should be utilized , WD-40 may be applicable as recommended by Tom in a pinch .

Foam prefilters are not oiled in all applications. None the less, I see you have an interest in how this works together. Might I suggest running your machine for 10 hours, filter treated/ not treated- pull the plugs each time and let us know if there is a difference. Also pull the element- is there any signs of a better filtration with the filter treated? There's your homework...

Just an adder Hogan even when recommended to be treated with Oil (Foam) pre-filters are only used to protect the main filter from premature dirt entrapment. The use of proper oil on the pre filter is only to assist in reducing of small particles getting threw the 1st stage of filtration , never saturate the pre filter rather just use oil sparingly . As Tom has said not all foam filters are designed for oil treatments , since they may impede air flow and cause a over-rich gas mixture . The Dealer advice normally is sound . Follow Toms advice and you can't go wrong .

Thanks everyone for your advice. I did put a lite coat of oil on the filter and then grabbed some paper towels to absorb any excess.

I think last year I just used too much oil.