Many engines don't get enough hours to change oil more than annually or as called for in the operating manual. In this case, as a pilot, I've been trained to change the oil before a lay-off so the engine sits with clean oil in it. I know some farmers let it set with dirty oil and change it just before starting to use it again. Do you have any recommendations on which might be preferable?
Synthetic lubes along with oil analysis when the oil sump is greater than 16 qrts. is the most cost effective way to approach oil change intervals. Petroeum based oils are not able to protect multimillion dollar equipment ie. combines and tractors like quality synthetics. Even if the applications are not multimillion dollar assets it still lmakes sense to use the most cost effective product options tht reduce operation and maintenance expenses and down time.
A big test for the best approach to what to do in a ie. combine or large tractor is applying a fogging product along with synthetics in every possible application, even hydraulics and transmissions. The attempt to refuse to use synthetics instead of petroleum based prodiicts due to the initial cost will not hold water because of obvious long term benefits.
I prefer to change before parking them. I've overhauled lots of engines and for those that set around a lot for long periods, you'd be amazed at how much crud accumulates at the bottom of the oil pan, on top of the heads, lifter valley on V8s and so on. Changing oil right after running them long enough thoroughly get everything heated up will at least get rid of the crud that's in supension. I'm sure you know that the word "crud" is a scientific name for that stuff that makes oil black don't you?