You can use plain non-detergent oil in them, but you should stick with the Cat oil for a number of reasons.
I started using a 3:1 mix of non-detergent oil and transmission fluid in General pumps after I'd repair them of whatever problem happened to be getting water into the crankcase. The transmission fluid would absorb the water and flush the crankcase clean, while the oil would maintain lubrication. After a number of times of going back to the same site and finding that the customer had not changed the oil again in a week as I had recommended, and the pump running perfectly smooth even years later, I started using it in all Generals. It might be an effective and more affordable option in your Cats.