Yeah, he's telling the truth. What you are doing is putting a different viscosity oil in the transmision. Will it cure the problem? Yes and no, the tranny will shift without grinding, but your tranny is still worn out. It's the same thing with the engine, when it starts knocking, you dump in a heavier weight of oil and the knock goes away, but it will get worse.

As my brother says, "sudden, rapid dissasembly of internal components"