I have just checked my differential oil and found that the rear looks pretty new, but I have some questions.

First, when I loosened the fill plug it sucked or hissed air. I know there is a vent, so I am wondering if the vent might be clogged.

Second, it was a bit foamy on top of the fluid and it had been sitting for at least an hour. Is that normal or bad?

There were signs that is had been very dirty before, the plug had a layer of dark, nasty, greasy stuff under the layer of light brown clear stuff that smelled like new stinky heavy weight gear oil.

I checked the front and it was grey and the fill plug was very dirty like it hadnt been checked in a long time. The rear plug had blue paint on it just as the transfer case had but the front differential had no (inspection) paint on it, just years of crud.

89 4x4 22re automatic extended cab. Thanksfor your replies.