I have used one for 4 years and never oiled it. Not sure how oiling it helps.
Just changed the spark plugs each year and sharpened the blade and that is it. Very good mower. Love it never change oil/oil filter.
We have had one for 4yrs and have always oiled it. I think oiling the air filter helps to keeps dirt on the outside of the foam filter. Making it easier to clean and allowing less dirt to get past the foam.
I just picked up an old Proline this year that had a very low hour deck and even less hours on the short block. The original owners manual had been sitting in the bottom of the owners trailer toolbox, the pages were stuck together. I went to the TORO website and was able to download the owners/parts manual for the mower and the Suzuki engine. The engine's manual recommends 5 drops of 30wt oil squeezed into the foam filter. They add a caution not to wring or twist the filter so it doesn't get distorted. I oiled mine, haven't detected any unusual smoke. In my experience, ingesting abrasives is about the only thing that will kill these Suzuki engines so I am going to do my best to provide it clean fresh air.
If it's running bad take the foam precleaner completely off and start it up. If it runs fine then it was too much oil on the precleaner. Too much oil will cause it to run rich meaning too much fuel to the amount of air. Make sure the oil didn't clog the paper element if so it will have to be replaced.
What type of oil are you using on the prefilter? Regular engine oil? I've used good ol' 10w30 before and did find it was easy to overoil the filter. Now I use Air Filter oil, the type that they use for motocross bikes air filters. A mechanic at a local shop told me that it's like a grease mixed with an alchohol carrier so it flows easily as you pour it and work it into the filter, then the alcohol evaporates and leaves a very sticky heavy oil on the filter that really grabs the dirt. The only downside is that the stuff is so sticky, about the only way I've found to really clean a filter off once you've used this stuff is with gasoline, so you have to keep an eye on the filter material to make sure the gas doesn't start to damage it. I use the the Yamaha (yamalube) brand if it makes a difference.
We run many toro 2- cycle mowers and we pour about an ounce of 15w-40 oil on the filter hes ball it up and squeeze it out a couple of times. The oil traps the dirt! No oil no more engine. We have never had any problem with them running too rich because of this but they will once the filter becomes plugged from dirt.
Get 4-5 extra filters $3.00 each and once they are all dirty throw them in the washer when your wife is not home it will make a mess hot water and a little extra soap. They will come out like new. Make sure to run washer an extra time before wife comes home to get the grass out.