You may use ANY Lithum or Lithium-thionyl battery with axial wires for
soldering. It does not matter if it says 3.0 V or 3.3 V on its label.
The size of the battery is proportional to its capacity/charge. There is
enough capacitiy in modern Lithiums for your Oberheim.


Lithium batteries DO NOT leak and will surely last another 20 years from
now. So Magnetic RAM is a good ideam but it is not an essential
improvement here.


Just make shure that the synth is not connected to common ground (via
audio or power cable), because the negative terminal of the battery is
connected to ground on the motherboard. Since you can expect the tip of
your soldering iron is also connected to ground, you may shorten (and
instantly drain) the new battery when soldering the positive terminal -
Or just solder the positive terminal first...

Good luck with the soldering iron.