This subject is discussed ALL THE TIME on many forums....but that's ok.
My 2 cents are: Once oil/lube is removed from its sealed drum or gallon jub and is placed into service - such as into a crankcase, tank rearend housing, etc. it is being exposed to oxidation and evaporating. Any crankcase has a vent to the atmosphere which typically allows for any offgasing when heated & cooled from operation. This also allows for moisture to enter and for portions of the oil/lube mixture to vaporize off. I am confident there are many times (epecially in the winter) that these lube systems do not get up to temp to clear out any moisture/condensation collect inside. So in considering this fact, regardless of hours ran, I believe it is prudent to change the motor oil & filter at least annually.
Some folks desire to follow the Operator's Manual to the letter. Fine - that's their decision. But I realize that these same manuals are very generalized - only varying between 'cold operation' and 'warm operation'. Secondly, as these vehicles/manuals age, they don't update as newer lubricates become available. My Granddad always told me, "Don't worry about what type of oil or when it was last changed, but just make sure it HAS OIL IN IT." In other words, what is important is to NOT operate it a quart or two low where it may overheat or suck air into the system.....
Rearend lubes, powersteering fluid, hydraulic fluid, etc. less often. If it's simple (like an external, spin-on fitler) I usually try to change the hydraulic filter annually, but only top off the hydraulic fluid. I try to bled brake systems annually, (and I recently will add hydraulic clutches to this annual bleeding). I test the pH of my coolants to track their acidity (keep above 8+pH). Grease stuff at least quarterly or as use dictates..... That's my 2 cents. Please carry on with other opinions.... :thumbsup:
BarnieTrk