How long does Mobil1 last if not used, but installed...

Mobil One is Dino oil and just like all dino oil about five years.
I had a bottle of "synthetic" or dino oil from a Auto parts store. the one with the red star on black. It was several years old, perhaps ten.
I poured it into my pickup when the level was low. I always use Mobile One but it makes no difference. About half way or better out here comes a slug, ofoil or grease or whatever, darker in color. I warmed the truck up good and drove it a ways and got the oil changed.
Shelf life of real Synthetic, I dunno and still ahven't gotten around to buying it and installing it in our vehicles, even though someone here gave me a link to a source, some time ago.

What's the worst thing to happen if you just leave it in?

What I might do is when changing the oil, have the waste oil tested at a lab. I wonder how much wear and tear is really happening.

Had a look at the Royal Purple I put in my Onan 2.5 years ago. It looked a bit used but not bad. I'll change it this summer when the RV isn't doing anything.

I have about 150 hours on that oil.

I run my PM longer than you - at least an hour a month - to get the condensation out.

Oil never wears out, But it gets dirty, water in it, corrosives from the combustion process from the gasses that leak by the rings. Moisture from cooling and heating and cooling and on and on. Oil has gotten expensive as evidenced by discussions in other threads on this site. BUT, oil is the cheapest thing you can put in that engine to prolong the performance that you expect from it. Don't jeopardize your investment to just cheap out on a few ounces of motor oil.

I change the oil and use full synthetic after every renaissance faire run where the generator is used constantly. During the busy use, I try to change the oil as per the manual. Since the oil sees a lot more "use" by a splash lubricated generator, I use full synthetic.

After the season is over and the oil is changed, I just let it sit. It isn't going anywhere, nor is it oxidizing with the air in the pan, so I don't worry about it until the next year. I'm far more concerned about changing the gasoline out (it gets the StarTron treatment) because bad gas is the first thing that will kill a generator dead.

Oils will list a TBN figure on their spec sheets.

Total base number.

The engine oil gets acidic with use and time. Time is related to the amount of condensation and condensation cycles. Moisture slowly makes the oil acidic.

Mobil 1 oils usually have TBN numbers close to 10.
TBN dropping is not linear. It usually drops to half or so fairly quickly, then levels off.

Blackstone labs will test for TBN if that box is checked. They recommend changing it when it drops to 2 or below.

If you really want to know how long you can go, send off a sample.

My opinion is it can go years. With short trip driving, It it not getting the oil upto 100c or so for at least 25 minutes that causes the TBN to drop because of not enough time to burn off condensation.

Short generator exercise runs might be considered as similar duties to short trip driving.

In a gennie, 4-ever.

For my blessed Hemi, I get the oil changed every 3,000 miles with full synth, and pace like an expectant father while they're doing it. For my gennies (3 of them), when they stall due to low oil I throw more oil in them. I think I've changed the oil once or twice, and it looked just fine so I stopped doing it so often.

As I understand these contraptions, there is no oil filter nor oil pump. They are splash oiled and all the crud just falls into the sump and sits there. While the crud is certainly altering the chemistry of the oil, the point of using synth is that it is highly resistant to this change in chemistry, or so I'm led to believe.

Either way it's a gennie, not a Hemi, and I've only got so much time on this planet. YMMV!

I ran my Honda mower for 5 years and never changed the oil. When I finally did change it, I sent a sample in to have it analyzed by Blackstone. They said the oil was clean, normal minerals etc, and that I could run it longer.

Oil is cheap, change it if you are worried about it.

+1 for Oldme, run it at least five years unless you put in some heavy usage.