It's a 100% petroleum product, which means it's "oil", and according to the TDS it's viscosity at 100C is 110cSt (in round figures SAE90 is 13 to 18, SAE250 is 48+) so it's a really thick oil.
Which means that adding it will increase the viscosity of your oil, and dilute the concentration of other additives that were in the original oil.
The shearing effect of gears makes oil become less viscous with age, so because adding this product will initially increase the viscosity of your oil (depending on the rate of addition) to way above it's labelled viscosity, the oil will take longer to drop below it's specified rating. So it "lasts longer". Although, of course, for most of it's life it is in fact out-of-spec by being too thick, and relying solely on viscosity to define oil changes is perhaps unwise.
I would assume that because it is a 100% petroleum product with a very high 100C viscosity, it probably (they don't specify) is very viscous at low temperatures, so your multigrade "W" rating will instantly disappear.
Adding this product, depending on the proportion used, increases the viscosity of your oil, and (probably) kills the "W" rating. Which is the equivalent of using (say) SAE190 or SAE250 oil, but possibly with reduced additives concentrations because of dilution. Yes, it will make the gearbox quieter. Yes the oil will stay thicker for longer. But you could achieve the same by buying SAE190, rather than 80W/90+LOS.
Now comes the real question. Would you put SAE 190 or 250 oil in your gearbox?

