Rear Diff Gear Oil Change

Yesterday, I drained and refilled the rear end in my '09 dually. This was the original fluid I took out (16,000 miles) and replaced it with Mobile 1 Synthetic. I jacked up each side for a time as I was also rotating tires. The quantity I removed was slightly more than 2 quarts. I refilled with 4 quarts which brought the fill level up to just below the fill opening. Was my fluid level low from the factory (no evidence of any leaks) or am I now filling it too full?

As a side note: I called the Mobile Oil Company and talked to the tech reps on what to use, 75w-90 or 75w-140, for my application which is carrying a 4k camper and towing a Jeep Wrangler. They said that for my vehicle to use 75w-90. They advised that the 75w-140 is too heavy a product for my vehicle and could blow out the seals. I know some here have recommended or considered using the 75w-140, so here's additional input from the Mobile "tech experts"???
Bob

I think you were dealing with a low factory fill. GM is notorious for under filling the diffs. Mine was 2 quarts low when I bought mine back in '01 and my brother's '07 was the same way.

Mobile 1 synthetic gear lube is top shelf stuff, and 75w-90 works well unless you're really towing/hauling heavy. Your camper/Wrangler combo probably weighs in the 7500lb range, so the 75w-90 is adequate in my opinion. I'm not a Mobile 1 tech, but I don't buy the 75w-140 blowing out seals.

Thanks Jeff for the response. I kind of suspected it was probably a little low from the factory.
Bob