I oil a Uni as if I'm spray painting it, making two passes on the inner element only: one pass from above, and the other from below, to get the pores from both angles. I also make a quick pass on the end foams just for good luck. I can't see any benefit to oiling the outer foam, but I could be wrong.
When you run out of the original Uni oil, as you likely know, most auto parts stores carry aerosol filter oil, or one can use regular old Dino based motor oil applied via a spray bottle. (600 million old cars & lawn mowers with oil bath air filters did just fine with this method ;-)) True synthetic based oil as used on an air filter may not combust as well as Dino would during a potential excess purge cycle, but it is probably of very minimal concern.
