Hey RobG!
The 660 Grizz will not be quiet with any kind of oil that you put in it because of the gearing design.I`am not sure about the rear end differential but I know that the front end is loud when running at top speed because of the design of the gears.The front has helical cut gears that are not quit when they are run at higher speeds,but are stronger than the beveled cut gears of the cars and trucks of today.The rear end gears might also be helical but they do seem to make as much noise as the front does.
The 660 Grizz will not be quiet with any kind of oil that you put in it because of the gearing design.I`am not sure about the rear end differential but I know that the front end is loud when running at top speed because of the design of the gears.The front has helical cut gears that are not quit when they are run at higher speeds,but are stronger than the beveled cut gears of the cars and trucks of today.The rear end gears might also be helical but they do seem to make as much noise as the front does.