@DB50 That's really dependent on where you live...but IMHO I don't think it matters...75W90 is a little thicker at higher temperatures and some will like that as they feel it's better protection for the gears. Others will agree with "affect drivability and fuel economy". And prefer the thinner oil.

So the ball is in your court...you choose what you think is agreeable for your usage.

 

Forget about those oils... no synthetic.
Phillips MP 80W90 is the oil to use in manual trans Subarus. 2 independent shops use it that I've been to. . it's a GL-5 oil.
I have it it my 2010 manual trans. Shifts great.

 

I used Motul Gear 300 75w-90 (note: NOT the LS, which is dedicated for Limited Slip differentials) in my '17 Forester with the 6-speed for going on 4 years now, and I've been very pleased with it. I noticed no difference in fuel mileage, but did notice that gear changes in colder temperatures were actually a little easier. No difference in the dead of summer.

For reference, I also used this same gear oil in my 2015 WRX with a 6-speed, as well. Even with 71k miles on it, it still shifted beautifully and was whisper quiet for gearbox noise.

Aside from the Motul oil being slightly expensive, I've had zero complaints.

 

I am trying an experiment. For no other reason than I hate the smell of degraded 75-90. Arizona conditions take their toll on gear oils. Silly I know. You can read all the "suggested" benefits of a syn gear oil. I will still be checking the condition of the oil. SYn is not an excuse to oil it and forget it. Last Diff oil change I used Triax Synergy Gear MAX 75W-90 GL-5.

 

Awesome, I went ahead and ordered the Motul Gear 300, it appears that particular gear lube was the one most people trusted for the MT. I had some Valvoline 75W90 I was gonna use, but it has friction modifer and all the forums said good for differentials but not so for manual transmissions. I tried getting the Subaru Extra MT, but unless I want to buy 5 gallons at the tune of $500, that's not an option, so I bought 6 quarts of Motul from Amazon. Thanks for the info.