I've been using
Cat Pumps since the Cat 400 (don't know the year), they went to the 410, then 420 then the 430. The 430 was is the same body as the 400, but just kept changing the front sleeves and pistons. When the 310's came out we shifted to the to that model. We never change oil til we have a problem that requires the removal of the pump from the platform. Usually it's because the oil seals have gone bad and the crankshaft of course starts filling up and changing the color to white.
When it's time for an oil seal change we strip the pump all the way down pulling the crank shaft. We then turn the sleeve over when we reinstall so it will wear evenly, put it back together and put it on the shelf. We have about 7 pumps sitting on the shelf waiting to be used and will probably outlast me!
I run my pumps @ 900 psi and have for as long as I can remember and use 4.25 pulleys which will give us (if my memory serves me correctly) about 4.0 gpm on the pump. They not working too hard at that pressure and that's why I believe I get the life out of the pumps that we get. Tip sizes are 4008 and change to 4007 during drought situations.