Bob
Usual hydraulic oil for jacks is iso 32 grade such as Mobil DTE 13M , Shell Tellus T32 or an equivalent.
Assahola
Its best to stay with the hydro fluid, Gear oil will only gum up the internals of the jack over time If it was a very pressing situation I'd say do it but If its not then dont Gear oil is a lot thicker ,stickier , its also hard on the seals and the valves too and it will work but as far as saying its a subsitute it is not ---
Anonymous
Hi floor jacks are hydraulic so need hydraulic fluid not engine oil. old engine oil has only two uses burning it. or painting horses hooves to make them look shiny.
?
Can't you just get a container of jack oil? I wonder why people always want to do unorthodox things.
If you need to add fluid, why don't you fix the seal??
Here's some reading material:
http://www.practicalmachinist.com/vb/general-archive/hydraulic-jack-oil-question-94274/
http://www.hyjacks.com/H7.HTM
Rolo
Nope, you can get hydraulic bottle jack oil at almost any good auto parts store.