Here's my thoughts and justification.
If want to go all the way, change oil 2x/year. Someone mentioned changing filter every-other time, I'd suggest against. Filters not that expensive relative to oil cost, but what's the cost of a filter becoming plugged and bypassing unfiltered oil? If you're changing the oil you must have some concern over contamination...
If changing 1x/year, I'd suggest spring. Typically see less use over the winter, more getting the machine out for a few minutes to move snow, or even an extended time, then letting it cool off, then warming it up again. Also in fall/spring castings submitted to heat/cool cycles just sitting. From what I've seen, this can result in condensation of water from outside contaminates inside the engine/trans/what ever. If you leave this in, will continue to cycle water contaminated oil through system when machine pulled from storage in the spring, and typical use suggests heavy use in the spring, either garden/field prep, or mowing chores starting. I'd rather have a fresh oil change to begin this use with, so I suggest a spring change.
I have a index card storage box at the back door. Each machine has it's own card. Each maintance/major repair gets documented on the card for that unit. I've sold things in the past, and new owner gets manual and at least a copy of the card. Documents repair history, somewhat of a usage report as each line item gets date and hours for units with hour meter. I've seen others that keep all units together in those small flip books, but I like each machine broken out.