replacing a cable once a month on two bikes would get awfully pricey!
ebay “ptfe gear cable”, I get mine for about £1.20 each when bought 10x at a time.
If middleburn don’t make cable oilers anymore, then brand-x definitely do/did.
Only problem is you then have to use unsealed cable ends, which let crap/water in, so they to a large extend only solved the problem they created as a full length outer has less points for water to get in). Best bet IME is just SP41, with the proper shimano end caps (which have o-rings in them) and some sort of shield to keep 90% of the crud off it (the long noodle ends work well, or old V-brake accordion thingies.