I recently installed a Mag-tech differential cover. And refilled with 75w140 synth. oil. Brand oil was Coastal, only sythetic I could find in town. After driving a few miles I rechecked level and it was low. I had no more gear oil to add. So I came up with the bright idea to add STP oil treatment. It says it can be added to gear oil. Spotted some oil under rear end. And so I decided to take a look.
I find oil is coming from vent hose. Do you think it is the brand oil I used or the STP causing this problem. I have heard of this problem with Amsoil gear oil in our trucks. Anyone else had this problem ? What did you do to correct the problem ?

 

Welcome to the club!

Jerry from Mag-Hytec said that he had some reports of foaming issues with Amsoil and fluid coming out of the breather tubes. He said 5 quarts is the correct level for their F150-9.75 cover and that the extra amount puts some fluid into the axle tubes. He suggested that fluid coming out the breather would not harm anything. Mag-Hytec sells some gear oil from Lubrication engineers and the fellow said he has not had any foaming issues with their gear-oil.

The Lubrication engineers instructions say to flush the differential with flushing oil before adding their oil. I think this may keep the foaming down. I just added Amsoil to mine without doing a flush and the detergents in the differential fluid may be foaming because a little of the dirty old differential fluid was still in the axle.

Here is a theory I have...

My breather tube is about 12 inches from the tire on the drivers side. Lets say I am driving down the road and the road tilts to the left enough and the extra differential fluid fills up the left side of the axle. Now the breather hole on the axle would be closed off with fluid instead of air. I increase my speed enough to cause more foaming and heat that causes the air in the differential to expand. This pressure then causes the fluid to expand up the breather tube.

I just let it spit out the extra differential fluid and forget about it. There is at least 3 or 4 quarts left afterwards and thats plenty.

A word of warning about the dipstick. Kansas dumps sand on the hiways during the winter and it gets all over the bottom of the vehicle. Wash off the bottom of the vehicle around the differential and the spare tire along with the axle real good and then take a cloth and wipe off the top of the differential cover before you remove the dipstick. Sand and gritty dirt can get into the dipstick threads and its bloody hell getting it to screw back in. I have to get a toothbrush to clean out the threads sometimes.