Oil, use the right spec and amount, change it regularly.
Folks get very particular about this subject and if you're one of those please look up the original specification of whatever machine you buy and follow the manufacturers recommendations (substituting modern alternatives when appropriate).
If you're like me, and I spent a lot of time building race engines, you'll be happy with a 5 gallon bucket of hydraulic oil from your local farm supply.
In the grand scheme our hobby usage is way, way less than any decent machine was built for and we take much better care of them than the factories that used them to build a nation.
Funny story, my daughter messaged me this morning because she had her car at the dealer for a recall and they wanted to sell her all kinds of preventative maintenance that was way above and beyond even what her owners manual recommended. One of the items was a clutch oil flush (her car is an automatic with hybrid powertrain). Yes, the manual said to flush this system, which apparently is used to engage and disengage the electric motor, every 22k miles. Really? Modern hydraulic fluid in a sealed system is going to go bad that quickly? WT.... I told her to just get the recall done and get the if out of there. In my job I maintain all kinds of equipment and have found that while maintenance is key for preventing failures it will not prevent every one. In my old business (outdoor power equipment sales & service) I told customers "maintenance is cheaper than repair and repair is usually cheaper than replacement, but nothing fixes just plain worn out".
So, my theory is if you want your machines to run for a long time you should use plenty of oil and change it whenever it looks bad, or maybe just every year. For me that's much easier to do with a $50 five gallon bucket than a $20 quart....
YMMV,
John
