I'm like you, low acreage use once a year, mine is about 5 years old now. I try to look at it in the spring, check the oil level, grease any grease points for bearings and grease fittings on the PTO shaft. General quick inspection for tine wear. In 5 years I've not had to add any oil, no real wear, just a little grease. Maybe this spring I will change the gear oil as I have plenty of gear oil in my shop, not sure it really requires a full oil change but I have also not really reviewed the oil condition to date.
Like you I store inside and I'm convinced that helps with equipment life and maintenance requirements. As commented above it can't hurt to change the oil. You also stated it was a used unit, I like to establish a benchmark on used equipment and try to keep a log of work done on equipment. I have my moments when I think I changed that gear box oil 5 years ago only to go back and look at the log to realize it was 15 years ago !!
I try to maintain what ever I can and normally do tractor maintenance earlier than recommended as my hours put on my tractors each year are low.
