Transmission Oils - Vista and Wheatland

I know that this topic has been discussed before, but I am still looking for some advice before I invest in a 50 gallon drum of oil. I am about to change the transmission/differential oil in a Vista and a Wheatland. I have always used Castrol oils and their recommended transmission/differential oil for for these tractors is Castrol AS Agri Trans Plus. This oil is a 20-30 wt oil, and I think similar to the Hy-Trans and Hy-Gard oils used and recommended by many. These oils all seem to have similar viscosities at temperature ranges from 40 to 100 degrees centigrade. And all are combination transmission/hydraulic/wet brake oils.
While many Moline manuals do not recommend a specific oil viscosity for the transmission/differential, the older manuals for the M5 and M670 do recommend that the oil should be SAE80.
Since the oil in the rear end of these tractors does not have any hydraulic function, and these tractors have dry brakes, should I use the combination type of oil or go back to an SAE 80 wt oil? If there is a benefit from using the thinner combination type of oil, what is it? And as these tractors also have transmission oil coolers, which type of oil would be more effective in dissipating heat? And which type is best for the TA?
Seems lots of questions I know, and maybe the answers will give me the peace of mind I am looking for. One point to note is that I am in Australia, and these tractors never see temperatures below freezing.

I'm pretty sure alot of people use hy-tran or hy-guard.

I agree with G1355. The lube pump wont be able to pump 80w oil. The M5s and M670s used splash lubrication in the rear end and the AT so the 80w oil worked in them.

I have never been able to get a good answer to this question either so I'll give you my opinion. I have used both types of oil in my Vista and it has the 3-speed ampli-torq. At the present time I am using 80W-90 and it WILL go thru the lube pump just fine and develop the correct pressure. I just think I get more gear protection for the final drives with the 80W-90 and we all know that that is their weak point. I work my Vista in the field about 200 hrs. a year and that may make a difference in what I choose to use.

I agree that this topic has been discussed so many
times... I think that if I were planning to do
anything with my tractor, I would put something
like 80-90 syn or heavier. Just my opinion.. run,
worked, repaired G1000"s for years... Don"t plan
on changing to lighter oil.

I think that the people that recommend hytran are
not the folks that put thousands of hours on
G1000"s like others on here... not saying that I
am one of them though. This is my two cents.

I agree, I do like the heavier oils as well but when I got my G950 with a bad lube pump I was told 80-90w wouldn't go through it, that is what I was going by. Good to know that it will.

Folks, Many thanks for all your responses.

This is what I will do and why.
I will use 80W-90 oil in the transmissions. I do think that in these situations, this would be a better way to go. I have not had any issues with the 20-30 hydraulic/transmission oil, and I have never seen it get overly warm. I do notice though that after a few hours work the transmission light flickers on when the engine is throttled back to idle. I don't think that this is a problem, but it must indicate that the lube pump loses some pressure on warmed oil. One day I will measure the pressure on both cold and warmed oil.
I also discovered something else today, and something I should have known. The MM manuals do not show the weight of the recommended transmission/differential oil The Part number for a 55 gal drum of this oil for a G900, Vista and Wheatland is 10P3290, whereas for a G1350 and A4T-1600, the 55 gal drum Part number is 10P3732. It makes me wonder whether MM changed the oil spec for the 50 series tractors.