I've personally never heard of foam filter oil. The Briggs & Stratton overhaul manual says engine oil - the same oil you use in the sump of the engine. It does eventually destroy the foam (after quite a few years) but a car-washing sponge only costs $2. If you know of an affordable oil that is kinder to urethane foam than engine oil is, that might be worth a look. The essential thing is that it coats the entire foam block, and then is sticky to passing bits of dust and grit.
