I have about 7. 5 qts in my differential, have since I put it on 7 months after I got the truck. I've never had seal problems of any kind. My temps run between 200-230 when I'm towing depending upon ambient temperature. You have a puke tube that runs out of the differential that should keep you safe should you see excessive temps, but you may have other issues if your temps get out of hand. I had a pinion seal problem on my truck that was a pain in the a$$. I ended up having my axle housing replaced under warranty. While I was having this problem I dropped the amount of lube in the differential to where it seeped out the sight hole in the Mag-Hytec. This is equivalent to the factory level, although the volume will be more due to the bigger cover. The pinion seal still leaked, so the leak from the seal wasn't due to an overfill condition. Theoretically, with the additional amount of lube, the bearings and seals should get a constant bath in fresh lube, extending the life of both. I guess time will ultimately tell if this is so. You should be OK filling it with 8 quarts. Opinions will vary on this, so ultimately it's going to be your call. Hope this helps.
John