Thanks for update OP.
Did you not 'overfill' RL this time round ?
I did change out the Amsoil MTG with the redline MT90 75w-90 and have been happy so far. The shifts are smother, it still sticks getting it in and out of gear but it's much better than Amsoil MTG ( that's been airated). The redline is a little thin for me but it seems to work, it does warm up under load but it's maxed out about 210*F going up white water with 8k in tow in the power in 6th (over drive) in 85*F ambient (And it did recover back down to 190 in about 10 miles.).
Redline MT 90 is not thin, it's thicker than Amsoil MTG at 45*C upwards and thicker than 5/3 blend at 50*C upwards, which forms close to 95-98 % (?) of your operating regimes.
It's interesting to note a thicker MT 90 operates 'cooler' than a thinner Amsoil MTG 75W90, so much for advocating otherwise often times.
Granted, physics in frictional drag losses is physics.
From this experience I'm 99% sure the original Mobil Delvac/ synchromesh mix was over filled with at least a quart of ATF from the original fill since I did that removal through the side inspection plate also BUT that held up much better to the abuse /airration better than anything else and still kept temps much lower.
Hence, the so called potentially increased 'aeration' ferocity attributing to 'overfill' ,has no bearing on increased rate of heat generation and fluid temperature increase in 'overfill' , if it matters .
All in all I think the Mobil Delvac SAE 50 transmission fluid/ Amsoil Synchromesh mixed at 5 quarts Delvac to 3 quarts Synchromesh is the best fluid I've ran so far. I did have shifting issues with it that where getting worse but I'm not sure it it was the fluid , or if it was some interaction with the left over ATF or airation??? This combo had the best protection, the best ability to keep temps way down, and fight airation.
As suggested by several posters above, you may want to consider MT-1 Mobil Delvac Transmission Fluid SAE 40.
In your operating conditions, I wouldn't be surprised if this Delvac 40 delivers you improved performance in temperature drop vis-a-vis 5/3 blend , I have no doubt it's more appropriate for your 'demands' .
At this point I will be keeping the Redline MT90 75w-90 in the trans as it's working good, it just does not keep the temps as low as the Delvac/Synchromesh combo did, to me this means less protection BUT does go in to and out of gear better than the Delvac/Synchromesh combo, so it'll stay for at least for the next 25k.
Yes, MT 90 at 15.6 cSt (210*F) offers far less components protection in relation to 5/3 blend at 20.0 cSt (185*F).
Would the suggested Delvac SAE 40 performs better than 5/3 blend above in terms of operating temperature and operating viscosity ? I would speculate so, if perceptible.
Would it perform worst than the 5/3 blend or RL in shifting ? I would think not necessary so, in practice. Only OP can demonstrate such.