General Question about Oil Changes

Hello all,
The mowing season where I live starts about late March/early April and ends by November 1. I typically change the oil in my mower (it's a Toro zero turn with a Toro-branded engine--has an oil filter) once a year. This is residential mowing only. No commercial use. I probably mow about 3 times in two weeks in the middle of the season and less frequently towards the beginning and end.
Question: when should I change oil? Mid-season, sometime in July? Or after the last mowing of the year? Or before the first mowing of the next year? Or does it matter?


Wrench. I help when I can
Personally, I think used oil should never be left in any sort of engine when going to be idle for many months.

As the oil will be accumulating carbon deposits, creating sludge from condensation, hence trapping moisture in the and about the molecules during the operating period and promoting particle sediment during prolonged dis-use. I find it is better to change the oil just after the extended use, before the hibernation period so to speak. New oil doesn't break down as fast as used oxidized oil when sitting over time.

Proof is that you can have pints lying around for years on a self and good to go. I wouldn't say the same for oil that is oxidized. You can see that if you leave some used oil lying around in the back of the shed, a thick layer has dropped to the bottom of the container. That is what will happen in the mower. Alot of cars that sit for long periods of time with used oil in them rot from the inside out due to this.

Oxidation, nitration and sludge are the worst things that can happen to an oil. So having that sit in your mower over the down time may lead to premature parts corrosion due to oxidation in the molecules of the oil. Oxidated oils will have increased hydrogen in suspension. Hence moisture.

Anyways, I am not a scientist, but from my experience better to have that crank case full of new oil over the winter than have that old crap eating away at the inside over the winter. I don't think the oil would break down in a sealed environment while not in use. So, there is my opinion. And as I say often, opinions are like arseholes, everyone has one.

Maybe someone else has another valid opinion that is worth listening to.
Switched to full synthetic oil in my small engines, wow, what a difference, air cooled, and synthetic can take the heat. Also add on ounce of Seafoam per gallon of gas, keeps the engine must cleaner. Also pay a little extra for ethanol free gas, small engines do no like ethanol, corrodes the main jet, just don't have problems.

Was around 1-15 years ago, our wonderful EPA made all small engines get rid of the mixture controls. Was at a small engine shop, bunch of people there screaming they couldn't start their engines. Owner said you should add fuel stabilizer. When their major problem was using ethanol, couldn't keep my big mouth shut. They don't clean the carb, just charge around 150 bucks or more to replace it.


Staff member
The flip side to that anti-ethanol rant: My wife went out to her mower a couple of months ago and started it right up with just a couple of yanks. It had been sitting outdoors in the rain since last fall, with E10 in the tank.

Carbs were getting very corroded back in the 60's, long before E10 could be blamed.
Lets face it, I doubt none of us REALLY know. But here is another thought. Some engines in old cars, motorcycles, and small engines have sat for up to 40-50 years with old oil and have started up with seemingly no problems. Go figure.
And, of course, this generation of manufacturers have extended the old typical 3,000 mile car oil change to over twice that amount.
Personally I don't think it will make much difference which route you go with on your mower. But, if you made me pick one, I would probably do like Nickb2, after the mowing season.