JMan: You should "check" the air every 50 hrs (more often if operating the tractor outside). If it is dirty, do a complete air change and replace the stems. If it is not dirty, do it anyway. Use only replacement air run through a compressor with a filter and dessicant cell (but not a lubricator). Don't have the valve stem at the 3 o'clock position if your tires are loaded w/ beet juice unless you are making Borscht.
When in doubt, don't pay any attention to your owners' manual. Those things were written by "airheads." /forums/images/graemlins/smile.gif
I think that that about covers it..... /forums/images/graemlins/wink.gif