Need help choosing correct fluid for manual trans and differential
 

I’ve been meaning to do my transmission and differential change on my manual FRS, I went to Toyota to pick up the fluids but they said they don’t carry it..(which is weird but wtv) Can I just use sat 300 75w-90 for both differential and transmission? I know the diff is LSD with NO clutches so additives not needed, but can anyone confirm? Is motul good for both ??

A lot of folks like Redline MT fluid. I have it in mine.

Sadly you can't use the same fluid for both. As was said, many folks run Redline (MT-90 for transmission, 75W90 for differential). Many other fluid manufacturers offer the right fluids for both.
If you do want OEM fluid and Toyota doesn't carry any, you can try Subaru (for a BRZ).

For what it's worth, I run Motul Gear 300 75w-90 in both. A lot of people do. Yes, I've researched all over if this is ok, good, bad, etc.

Motul Gear 300 is weird. It's rated GL4 and GL5. The issue comes up because you shouldn't use GL5 in the transmission as some of the additives can "eat" the brass synchros or wear them out over time.

Thus, using a GL4 only rated fuel is a better bet, hence many recommend Redline MT90 as it's a 75w-90 that's GL4 rated only.

So you can really get into the chemistry make up and argument side of things since the Motul is oddly rated both GL4 and GL5, not just strict GL5. Like what does that even really mean? Technically speaking, it meets both specifications. But if it's GL5 rated, it must have the additives, but it's GL4 rated it's also ok. So which is it?

For what it's worth, I searched high and low for any failures from running Motul 300 and haven't found any. Tons of people are running this in the twins and many more in other Subaru transmissions with brass synchros.

My assumption is Motul wouldn't list it as GL4 backwards compatible if it damaged brass synchros. I did find on some listings just now that it says "Suitable for any type of seal and yellow material used in gearboxes design."

... so I guess I just answered my own limited remaining fears.

To add to that, the 2017+ facelift updated the fluid compatibility to GL-3, GL-4 as well as GL-5.

My take is that even if the fluid is practically the same, I'd still rather use the recommended fluid (by the fluid manufacturer in their "find my fluid" search) for each use. You can save yourself one quart/liter bottle when using the same fluid for both, but that's about all the saving you'll get. The potential for damage by using the "wrong" fluid is not-zero, IMO, and outweigh the cost of one bottle of fluid for me. To each their own, though.

That is very interesting to see the later model manual mentions GL-5. I wonder if there was any sort of update to the trans itself.

Either way I right straight up GL4 in the trans amsoil and GL5 in the diff gear 300LS. Works great year round street and track

Motul Gear300 for both fills has been fine in my FRS over several years, road and track but very low total mileage. I change it every two years.
Note: my FRS isn’t winter driven so COLD shifting hasn’t been experienced. I can say that Redline was the favoured go-to back in my Yaris (Aison 5sp) days.
Much discussion at YarisWorld.com if interested.

I prefer the way the car shifts with Redline. It doesn’t matter so much now that it doesn’t get driven on the street. Plus it is much easier to get.

Concur.

I've ran motyl gear 75w90 my last two changes. Last full had a bit of redline mixed in the fill bottle.

False.

I have tried 75w80 fluids from Motul and Redline, Motul one made shifting worse when cold, I assume it would be worse with thicker fluid. Redline was much better, and not sure if it is a placebo I like it when it's warm as well. Next fluid I am going to try is Ford Fluid XT-M5-QS, it's called unicorn tears by miata people (NC and 86 transmission was based on Aisin AZ6).

That's interesting. I know we are talking about a 1st gen here, but I'm looking at the manual for my 2nd gen (and I was pretty sure it's the same TL70 transmission):

Gear Oil LV 75W (Redline makes an MTL LV that is 70W75 GL-4) their site lists it as a direct replacement for "Toyota MT Gear Oil LV 75W" which is the factory fill.
API GL-4 SAE 75W

Differential Fluid:
Differential Gear Oil LX (Redline equivalent is their 75W85 GL-5 gear oil)
But API and SAE rated GL-5 in 75W90 are acceptable.


All that being said, I'm getting the transmission/differential fluid changed in my '23 tomorrow (about 4600 miles), with Redline MT-90 for the transmission and Redline 75W90 Gear Oil for the differential.

As I understand it, the 2nd gen has lower viscosity fill from the factory. They did make some updates for the transmission, even if it's basically the same unit. AFAIK this is to improve emissions/gas milage a bit.

I honestly trust the fluid manufacturers' recommendations (i.e. fluid choice based on the vehicle info) as they did all the thinking about which fluid matches what spec. Second choice is wide vehicle-specific community recommendations.