I think that is a huge debate among a lot of "car" people. Some say you can go up to 7,000 before changing.

But if it helps you any, subaru is chaning the oil in my car for the first time at 3,700 miles. Supposedly the need to change the oil after 1,000 miles no longer exists with newer cars, or so I'm told.

I plan on doing my own oil/filter after this every 3,500 - 4,000 miles.

 

The manual says 75W90 works for all temperatures but it offers other options for your specific climate. Take a look in the manual.

I just changed my 02 WRX Wagon 5MT gear lube and rear diff with Mobil 1 75W90. It does indeed shift noticeably better, but it does foul with your synchros a bit. There is a lot more chance of grinding when shifting "lazily." Like at a restaurant drive thru or stop light, when you need to move forward quickly, when I hit the clutch and immediately go into first, it grinds. If you push in the clutch and wait a few seconds, it's fine because everything in the tranny has stopped spinning.

Overall, I'd do it again. High speed starts and going through the gears normally is great, but there are just a couple times I've noticed the M1 is a bit more slippery than the syncros are designed for.

 

BTW, I changed mine at 55k miles. I think the service bulletin says 60k?

 

I believe this information can be found in your owner's manual.

IIRC, severe duty cycle is a 15K mile change interval for manual tranny, and 30K change interval for normal duty. It's not that much more difficult than doing an oil change, so I've done it in my car every 15K.