Final Drive Royal Purple 75w90

Just changed the final drive oil to Royal Purple 75w90. I'm not sure what was in there before but since the engine only had 6000 miles on it I'm guessing it was the factory oil.

Before the change I was hearing some moderate gear whine at part throttle. After changing to Royal Purple the whine is completely gone so I'll call that an improvement.

It is so easy to change the final drive oil that I do mine every 2nd oil change!

Honda recommends 10-30 motor oil, however.

Yes I know what Honda recommends. It's a simple gear train with no pumps or oil galleries so running the thicker oil shouldn't be a problem. Plus I'd much rather a lube that is specifically designed to deal with high pressure shear loads.

But most importantly, I already had it on the shelf:)

Kenny,
Any chance the heavier oil will effect fuel economy? How about creating internal pressure that could cause oil leaks?

I doubt there will be any difference in either. 90w gear oil is the same thickness as 40w motor oil. The main difference is additives that allow the oil to handle shear loads better. Motor oil has additives that allow it to handle combustion byproducts that aren't relevant in a gear box. And since an engine oil system is pressurized motor oil doesn't need to deal with shear like gear lube does.

I would also say that Royal Purple is significantly higher quality than Hondas recommended Dino juice.

I have used both 10W-40 ( I am near Phoenix after all) and 75W90. Currently running the gear lube. Have seen no difference in drivability or fuel economy...

That said usually I use same oil as in engine because 3 quarts will more than do both.

Yeah in all truthfulness it probably doesn't matter much what goes back there. The gears aren't particularly high speed or high load.