Replace fluid in cyclops fluid trainer?

I have an old Cyclops fluid trainer which has lost much of its resistance. Even when you spin it really fast it doesn’t get harder, in fact it seems easier the faster it goes.
I am not the original owner and am in New Zealand so return to Cyclops is not viable.
I’m thinking of trying auto gear box oil or engine oil with quite a low viscosity.
Is it possible to replace the fluid in it? I don’t really want to undo it and find the old fluid spills everywhere and is difficult to replace as its ok as a trainer for kids to practice on as it is.

Cheers

I don’t know what the viscosity of the original fluid is but I would contact Saris before cracking it open. At the least they should be able to tell you what kind of oil to use, they might even send you a replacement unit. Which I would imagine would take a while to get there, but you never know.