One would really be spitting hairs one way or another. Sure, it's great to get the gunk out with the oil. A good oil will take it out with it when it is changed. Change the oil at least once a year at or before the PM schedule suggests, you'll be fine either way. Myself, I treat and run the fuel out of the carb at the end of the season and change the oil after I've run it in the spring. I then complete the rest of the PM and everything is fresh and ready to go.

I'm a "end of season" kind of guy. Since the mower is going to sit for 3-5 months, I don't want any acids and other combustion byproducts doing any potential damage.
I also feel any suspended "solids" in the dirty oil will tend to settle out and are more likely to "stick" to the sump over an "extended" time period.

Something else I do (actually after every use) is to pull the starter rope until the compression stroke starts.
This "seals" the combustion chamber from the atmosphere and relieves as much tension from the valve springs as possible. It also prevents a valve from sticking open during storage.

Bill,

That sounds like the reasoning I heard before. But does oil go bad over the winter. They always say to change the oil in your car in x miles or by x date; whichever comes first. Is there an expiration date on oil?

The manual also says to take out the spark plug, pour in an ounce of oil where the plug was and slowly pull the starter rope a few times, them put the spark plug back in loosly. I do this at the end of every season as well. I'm not sure I'd know when the compression stroke starts though. Is there any way for you to explain how I'd know when I'm there?

Its not the oil going bad over the winter, it is the cumbustion byproducts remaining in the oil over the extended time period that is a concern. I use synthetic oil now in all my equipment because it is less suseptible to these factors.

Just read your manual. They all state to change the oil at X hours (usually 25) and/or when storing the mower. Actually, consider doing this: when it comes time to put the mower away for the winter or any other reason, run the gas out of it - while it's still hot, dump out the old oil, but do not add fresh oil. Put oil in the cylinder and leave the plug hand tight with the wire off and grounded (if you have a grounding post). At this point you can turn the mower upside down or any other way you want to clean the deck, change blades, grease the trans and wheels or anything else. Add the oil in the spring when you tighten and connect the plug and you'll be GTG. The only possible drawback is if you forget to add the oil before starting. You could stick an oil bottle between the handle and engine/blade control bail and tie it in place for a gentle reminder! Then you can't start it until you remove the oil bottle...oh yeah, that reminds me...the oil!

Saxman, I started out changing my oil every 25 hours like the manual said. I had to quit this practice because it got a little expensive...

"I'm a "end of season" kind of guy. Since the mower is going to sit for 3-5 months, I don't want any acids and other combustion byproducts doing any potential damage."

I agree, and I do put the fresh oil in then.

Walt Conner

Oil isn't going to go "bad" over the winter.
Look at all the people that never change their oil and stumble through life without consequence!

Forgot=

The compression stroke starts when you feel some resistance on the pull rope.
Typically, when these engines are shut off, they'll "bounce back" from starting a compression stroke, so you only need to pull the rope about 1/4 turn or so.

saxman, thanks for the good suggestions. My manual says once a year.

Thanks to everyone else as well. I think I'll just make it easy and do what saxman said, but also put in the new oil at the end of the clean up (ie. the end of the season). Knowing me, I'd somehow forget to do it in the spring.

Now, is there any benefit to doing it twice a year? Also, when I let the mower sit for a couple of weeks, I notice a big puff of white smoke that quickly dies down to clean exhaust after a few seconds. Any concerns there?

Bill kapaun, ok so I should be able to just pull the rope a little into some resistance (about 1/4 of the distance that it would take to get back to non-resistance)? Won't it just bounce back to the beginning of the stroke when I let go of the rope or move the mower a little?

White smoke for a few seconds is considered "normal".
Vertical shaft engines are more prone to it than horizontal. Think how close the bottom of the cylinder is to the oil level in the sump. Having the oil a bit overfull exacerbates the situation.
You'll also have a bit of oil seep past the valves.
It doesn't take a lot of oil to make a lot of smoke!

Just pull the cord slowly.

around here we get wild temp swings and this results in more condensation in teh engines. so i usually change mine shortly before the end of teh season AND at the start of the season. of course, i also put tons of hours on my machines as well, so it ain't like i am wasting oil.

last spring i checked my push mower prior to first use. there was a ton of water in teh oil from teh condensation. we stay above 70% humidity almost all the time, and at times in teh upper 90% range. if your area is a "dry" climate, then you may not get condensation as bad as we do.

Thanks. I was thinking that having some oil in there would reduce the space for condensation and protect the interior more.

MD is fairly humid. We have very unpredictable weather as well being on the border of North and South. Changing the oil twice might be cheap insurance and environmentally better than throwing a whole lawnmower in the trash.

I change the oil in mine and then mow one last time. That way the "clean" oil gets a chance to move around inside the engine a while. I have had quite a few mowers and never had one go bad due to oil related reasons. I would be more worried about your fuel then about your oil. By the way, I fill mine to the tip top with fuel containing Stabil when finished for the season and leave it until Spring.

"By the way, I fill mine to the tip top with fuel containing Stabil when finished for the season and leave it until Spring."

I USED to do that, but changed this year-
In fact, I was a strong proponent of that method-

It seems with the mandatory ethanol in the gas, it'll still absorb water even though full.
My engine ran rather crappy until that first tank was used up. First time in 21 years with that mower (Honda).

Luckily ethanol addition isn't mandatory here (yet), but they are not required to label fuel with ethanol in it any longer (at least up to a certain percentage) so we have to be careful.

Sta-Bil's new super-concentrated (blue) marine treatment for ethanol fuel is great, but to realize real benefits you need to run it all the time. Many owners make a huge mistake by just putting it in the gas tank at storage time. This does nothing except preserve the fuel in the tank. It needs to be in the carburetor. The easiest thing is simply to put it in your gas can before filling it, every time. Then you can forget running it dry or adding anything at storage time.

Can you add too much Sta-bil. I usually fill 2 5 gallon cans with gas and Sta-bil which I use in my 4 cycle machines. I mix two 2 gallon cans, one 16-1 for my old Lawn Boys using LB oil(which says it has a Sta-Bil type additive in it) and one using Opti-2. Do I need to add more Sta-Bil or is the amount in the straight gas enough.

In the amounts of stabilizer you are talking about, it is fine. FYI, you can use too much Stabil though. I had a customer that used a bottle per 5 gallons used in his WPM - turned the casting of the carb pink! I ended up replacing the carb in the end.

Thanks for all the advice. To summarize my plan (more for my records than anything). I will:

1. Add fuel with Stabil at every filling
2. Change the oil before the last mowing to allow clean oil to move around the engine (Warm up the engine first)
3. Run the mower dry after the last mowing of the season
4. Put 1 oz of oil in the cylinder and slowly pull cord a few times.
5. Leave the plug hand tight with the wire off and grounded
6. Pull the starter rope until the compression stroke starts (1/4 slow pull into resistance and slow release)
7. Change the oil again at the beginning of the season after the first mowing.

A little overkill, but cheap insurance!