I agree with the others...don't do it.
I don't agree from any experience of renting anything out, I've got experience being the one you don't want to rent to!!
Before I bought my own backhoe/loader (JCB), I had never run any kind of equipment like this.
building new home, we rented a skidsteer. Now... my brother in law HAS owned stuff like this and in fact, several years after this event, actually got into the rental business himself (dozers, bobcats, lifts...)
We had my rented bobcat near my retaining wall, it needed refuled. We climbed a step or so up on the wall to refuel it not realizing that when we took the "fuel" cap off, it was actually the higher hydraulic fluid cap.
I think we put in about 10 or so gallons of diesel into the hydraulic system before we caught what we did.
I don't know if that hurt the machine long term or not but we ran it a while longer and then got it back to the rental place (running)
Our intent was good/nice, we just pulled a stupid moment when we did that.
(we did try to siphon the gallons back out and I've got NO idea if the diesel would have readily mixed with the hydraulic fluid or if it would have sat on top)