The oil lamp grounds through the oil sender. Therefore this description does not make sense. I'm not saying "it isn't happening" I'm just saying a common cause is not obvious. The path for the oil lamp is key switch---to dash cluster power at the pin on the PC board---to the oil lamp--through the lamp---to the sender terminal on the PC board connector pin---out the sender wire--out the bulkhead connector---to the oil sender---to ground via the engine block
In theory the voltage to the cluster should not be affected, because the power for the horn relay contacts and coil comes from a different source/ path.
"I don't know"
