I just put aftermarket Diff covers on and have a question. Do I stay with factory recommended capacity or still fill to it leaks out of fill hole? If it matters they are Solid brand

 

I think if you fill it with the recommended amount it will take you to just below the fill hole. I usually fill to the point of, if I can touch the fluid with my pinky in the fill hole its full.

 

Most aftermarket holes have a raised fill hole so this doesn't work.

And the factory fill amount is a good starting point but may have more interior volume so you might need more fluid. It's all about the level inside. Get it to where the factory fill hole is, which is like 1/4" over the axle tubes in the rear. Keep searching, there are good threads on this.

 

What's important to remember is the diff oil also lubricates the outer axle bearings in the rear, and the carrier bearings in the front. So, the oil level has to be above the bottom of the inside of the axle tube.

Here is a pic of the level relating to the stock cover. You can make a little dip stick to check the oil level with your aftermarket cover.

 

To take Ron's statment a step further, remember that rotating the housing for pinion angle effects the level too. There are times that, to do the job correctly, you'll need the axle higher than level to get the proper fill as the correct fluid level should be above the fill hole when the vehicle is level.

There is no set amount of fluid required to fill. Various differentials displace more fluid than others and will effect the fluid volume.