Hydraulic oil for older Yale & Presto electric jacks - how critical?

[richard newman]

Recently acquired a used Yale electric pallet jack and Presto electric stacker. Both need some oil, I'm wondering if It's important to use what the makers recommend or will any quality hydraulic oil do?

Also, as I have no idea what's in them now, can I just top them up or is there a problem with potentially mixing different oils?