I've always done mine at the end of the season, immediately after the last use.

It seems the general consensus is to do it after so you don't have contaminated oil sitting for months.
Combustion by products can develop acids etc.
It probably doesn't really matter though.

The longevity of the mower engine will not be affected either way. For those that take their mower in for a complete service- the overwhelming majority have the preventive maintenance done in spring. For many, it could be chalked up to procrastination- but really for most it makes sense. If they are having maintenance done including fuel system service- it makes sense to start out fresh. Keeping fuel treated and care used during storage will get you further than anything.

One might make a case that delayed preventative maintenance becomes a mandatory repair.

Thanks for reply. I've done both,I think I will go with after last mow. I treat my fuel than too.

Have it both ways; run it out of gas, then while still hot, dump the old oil. Pull the plug wire off and hang a tag on the handle to make sure you oil it before fueling in the spring.

I have always done exactly the same as saxman for the non-fuel pump machines. I never leave gas in the tank or carb. The two tractors; I take a chance.

Wisdom from the oil changing nerd! Don't want to hear it, then DON"T read it!

I change my oil enough it don't make difference. IMO problem is when you don't change the oil for year or more. Just adds to the problem you already started. Course then their the ones that don't know where the dip stick located another problems added. Adds up to BOOOOOM.

IMO it's wise to at least change the oil in air cooled engine twice year regardless of hours. Do that along with checking before you jump on the machine and you won't have oil related problems IMO of course

Course I'm going to here from the ones that say I only have 40 hours on my oil and it only been year and half??? Or I not going to waste oil changing oil that much after all Quart of oil 3.50 what do you think I'm made of money and want to make the middle east richer and use up all or natural resorces! Blaaa Blaaa blaaa I heard it all. Months of mowing in Dusty hot conditions take toll. At least I know My mower not going to blow up cause I didn't change the oil enough.

Oil purists seem to not to want to leave oil in the sump when not in use- many times on the theory that due to the different metals in an engine the oils become acidic or electrically change. That similar argument is made for changing oil pre-need. All in all though, it's a lawnmower engine. There is no wrong answer. To each their own. If oil is reasonably maintained- the wheels will break off the thing before the engine dies from a lubrication failure! ;)

Not sure if I should start a new thread but here's another end of season question. How many people drain the gas as opposed to gas treatment. I've done both,not sure if one is better than other.

If you drain it, get all of it out & run the carb dry.
IF you leave gas in it, fill it to the brim to remove all dead air space.
I do the latter, using stabilized fuel for the last couple mowings (never exactly sure when the last one happens until a couple weeks after it happened) to ensure the stabilized fuel is in/through the carb.