They're all the same part with different plugs. As long as the universal you installed was a pre-cat model it should have worked. In fact, most of the later BMW sensors are a direct exchange, only with different length pigtails. If you want to get a genuine Bosch that works, any '90s Ford V8 four-wire pre-cat sensor is the exact same part with a different connector. They're usually about $50 instead of the $120 that BMW wants.
The parts guy is trying to help by telling you what works most of the time. He's right, often older FI cars are just an O2 sensor failure. Our M42s have a more elaborate idle control, so you often have to do a bit more digging. Code scanners aren't a great way to diagnose older cars, the codes in our OBDI are very limited compared to OBDII.
Rough idle and O2 code failures can also be a result of vac leaks, bad ICV, miscalibrated AFM, bad temp sensors, plugged fuel injectors, bad FPR, the list goes on and on.
Also, long shot here, but do you have any silicone sealant on the intake or exhaust manifolds? Unless it's marked 'sensor safe' it will kill O2 sensors.
The radio had nothing to do with it.
