OBERHEIM SYNTH

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.