Toyota recommends changing engine oil on a Prius every 5k miles if you...

Here's a related thread in Prius Prime about mandatory running of the ICE to purge engine oil of the contaminants. Toyota built-in a mandatory ICE run cycle in case normal use does not get the engine to full operating temp long enough to clear out contaminants, Message: Restoring Engine Oil Conditions? | PriusChat

 

Too, I wouldn't assume that a very occasional engine run is going to put the engine oil in the same condition as a similarly driven regular Prius.

 

Wow. Here I am thinking of extending to 20k w the new 20k/1yr Mobil 1 oil....

 

That long interchange oil is super scammy, the filters won't last that long anyway...

The problem is as already stated the blowby watery mess that gets pulled into the motor. If you set up an aftermarket oil separator you can see how nasty the oil gets on rainy humid days. A hot motor turns this mess into steam then that goes back into the intake were the flammable stuff gets burnt off and the water goes out the tailpipe.

The biggest problem with the oil mess is that is makes a corrosive acid as it degrades the oil. The acid eats metal slowly.
This is problem is some what negated by using synthetic oil and or letting the motor get up to full temp for a little while. 5 minutes may or may not do it depending on the season. Cold winters probably not, warm summers probably yeah. (You could graph it with your seasonal gas mileage).

Taxi engines stay running longer and there is an argument to be made that these changes are less needed it that case. I have seen dozens of v8 taxis over the 400,000 mile mark not getting oil changes as a non taxi