Generator oil change, now or later.

We have the factory installed Onan 5500 propane generator. Changed oil and filter after 10 hours per manual. That was 18 months ago. I run generator periodically and now have 38 hours. With such low usage, since it's been 18 months, should I change the oil? Or, when?

The Onan manual says to change the oil and filter every 150 hours, but at least once per year.

If you don't use it enough to get the hour intervals I would change it yearly, preferably at the end of the season.
Fresh oil for the winter sit time is a good thing for your small engine.
Clean oil sitting through out the bearing surfaces instead of contaminated oil potentially degrading the bearings.