Yep agreed.
Gale Banks has a great video showing this. They made stock diff covers and molds of popular aftermarket ones out of clear acrylic and stuck heat sensors in and put them on a dyno with a camera and a thermal camera showing how the various covers did with the various oils and such.
The conclusion was rather shocking. For example, the cover that kept the diff the coolest and had the best flow. The factory. And this was a consistent across various axle types. The Dana 60, the gym 14 bolt, the Dana 80 etc. Even over his own diff cover.
The conclusion was that unless one was running a pump, all the aftermarket covers that had capacity, the cooler oils stayed at the back of the cover and the rest basically whiplashed and aerated instead of cooling.
I’ll see if I can find the video but it definitely was cool