Nice example Bear. Although you used Gear oil for your example, we could easily substitute 0W30 in an engine that calls for 5W20, with no other options. I get the point. It would be a long drawn out uphill battle.
Stick to approved fluids, and mfg recommended intervals, in the suggested grade, and don't play games while under a warranty. Sound advice, and exactly what the service writer told us when we had my GF's Jeep in for a warranty issue. He flat out said, DON'T USE A 30 GRADE OIL IN THIS JEEP! IF THERE IS AN OIL PROBLEM WE WILL PULL A SAMPLE, AND THAT 30 GRADE WILL BITE YOU IN THE BUTT! IT CALLS FOR 5W20, USE 5W20!