i bought some valvoline synpower full synthetic SAE 75-140limited slip gear oil (extreme) today because i want to replace my gear fluid tommorow, this stuff good or bad?

Not really the right stuff, your manual specifies either 75W90 or 80W90, best to stick with those viscosities. Also you do not want to put any gear oil containing limited slip additives in your transmission as they will interfere with proper synchro operation. Those additives are for use in wet-type clutch plate LSDs which you don't have.
Get the correct GL-5 oil in the recommended viscosity, either synthetic or organic.

ah ok, i will run to advanced auto parts tommorow, or walmart(all i have here for auto stores, we have about 85 restaurants lame) what do u recommend me to get, i want to change it before my mini road trip tommorow, any link on how to change it, not quite sure how to get rid of stuff in there, drain rear diff?

As I said, follow the recommendation in your owners manual which specifies a GL-5 rated oil for the transmission and diff.
If you're in a hurry I would suggest leaving it alone for now and dealing with it when you get back from your trip. If you rush and grab the wrong oil (as you already did) or make a mistake when changing it (i.e. drain diff before removing the fill plug and then find out that you can't get the fill plug out), you could end up with an undriveable car. It isn't critical to change the trans/diff oil like it is with engine oil, in fact in the maintenance schedule Subaru only specifies regular inspection of these fluids, not replacement. That's not to say it's a bad idea to change them but it may not be necessary as long as the oil is at the proper level and not contaminated.
Take your trip now, and do this job later when you have more time.

suppose i will do it when i get back then, was going to change because of frequent gear scraping action.

It can be difficult to find a 75W90 or 80W90 that does not have limited slip additives. But, keep looking and make sure you follow Mulder's advice. Any oil that says it is applicable for limited slip differentials is no good.

I ended up using Exxon Superflo 80W90 since it does not have limited slip additives.

-Jeff