Wikipedia tells me this...
silicone oil mostly siloxanes like polydimethylsiloxane

Other sites say...
Oil filled radiators are not designed to be refiulled
The oil used tends to be expensive and difficult to obtain
Air space should be left for expansion

I say this...

If it's leaking then unless you can fix the leak then refilling would be futile.

Amazon will sell you a new 2kW radiator for £36

Good Luck...
February 2017