A better question would be why were you considering their gear oil as opposed to others?
Here is a teaser look at how bad RP is in this test, change in visc and shear test. Shear, metal coming off metal under stress most likely because they are loosing visc which makes the oil thinner thus looses protection. Also, like I said I have heard guys from the 4x4 forums I know through the forum have to swap gears from failure after putting RP in. You may get away with a pavement queen, but you can't fake it for guys who get down.
Redline is the way to go, they don't fake anything or even promote themselves appropriately, they just make gear oil that will keep a low temp, stick to metal by polarity, stay clean and good film strength period. Esters may be optional for engine dynamics, but for gears and tranny's they should be considered a necessity IMO. There is only one USA company committed to esters the only true synthetic oil, and that would be Redline Oil. Other companies may use esters, but nobody like Redline uses them so much in every formula. Esters aren't made from the ground oil, they are made from mixing acid and alcohol, so there is no oil in that oil, only synthetic goodness.
To be fair and honest, here is a big study on gear oils and Redline doesn't come on top. The reason, Amsoil determines what the values are, so of course they will figure a way to "win". But it shows a lot of oils and how they perform in Gears, and I still suggest redline is the best but it would be a long conversation to why. Bottom line, they can't even test redline at high pressures because the test level ends while redline is still working. Many applications you can't even use even group 3 oil in, high pressures and loads require ester based formulas.
PDF larger study gear oils.
BTW, in some other tests even that Amsoil has done. Redline tests off the charts in crucial areas of performance like pin and V block testing and Falex pressure tests. And they are the only oil to do so.
