GC is a poor choice. Stop looking at numbers and look what is actually specified for the tranny. You'll destroy your syncros if you use motor oil. It's call "motor" oil for a reason.
I use Amsoil MTF in my RSX and it does wonders. The shifter becomes a toggle switch. Almost no resistance going into any gear. Heck, I can put it into 1st gear on the highay. It's stupid to do, but you can do it. Royal purple and redline have simular products. You only need 3 quarts, don't be cheap.
I'll tell you what, you put motor oil in your tranny, and I'll put purpose made synthetic tranny fluid in mine. I'll enjoy my effortless shifts and decreased syncro wear and you can enjoy notchy shifts and syncros that are on the verge of becoming molten due to a lack of friction modifiers. You only need 3 quarts. Don't be cheap. You are looking at $20 for Redline, Amsoil, or Royal Purple MTF or hundreds in repairs because you decided to take a risk. Is it really worth it? Leave the stock fluid, or get something better. Motor oil, no matter how good, is a downgrade. It is a bad idea. I repeat, BAD IDEA.
Motor oil contains all kinds of additives that don't need to be in a tranny. How much fuel dillution happens in a transmission? How about acids from sulfur and nitrogen? Oh yeah, that's right, there is no combustion in a transmission. Tranny fluid can also use tons of anti-wear additives that motor oil can't because of emissions and such. Use a purpose made fluid, I will say no more. End of discussion.
"syncros becoming molten due to a lack of friction modifiers"
Wow, did you miss something? I want a product that has mimimal FMs... hello, because that's what syncromesh fluids call for. What about the part where I mention the OWNER'S MANUAL allows motor oil in the trans? Does that not factor in to your thinking? How is it that I'm being cheap? I'm looking for the best product, and unless you've tried GC in your trans, how can you jump to conclusions about how it would work? A funny side note, I've actually read specualtion that the latest Syntec Syntrans is the same as GC.
Whew, don't get so excited, especially when you are missing info big parts of the subject.
The syncros need FM's to engage and match the cog speeds. If not, they overheat and prematurely wear out. I've given you my two cents, do what you want. Is there any verification that the Syntrans is GC? They might have the same base stock, but why even consider it? Just use the Syntrans or one of the other purpose based fluids.
FYI, I would never use GC because of the possible problems that would be out of pocket.
The best protection is going to be a purpose made fluid from a reputable manufacturer that meets or exceeds factory specifications. GC is a motor oil for motors.