The owner's manual (who reads that old thing?) discusses using the tranny for engine braking.

Unless you're doing serious RPM drops, you shouldn't have a problem.

I do it all the time, in moderation with braking. I also use it to slow myself on declines when I'm out running the hills. If the tranny breaks, WGAF? I doubt I'll own the car long enough to see it happen.

 

I used to beat the shiat out of my old Celica GT 5MT. I'd downshift/enginebreak from 5th to 3rd (it'd jump up to 8,000 RPM) to have speed coming out of the corners, with no problem. I put 240,000 km on it, (approximately 100,000-120,000 miles) without having to change the clutch or having any tranny/engine problems at all. And this is on a 1.8 liter 4 banger. I doubt the G35 would have much trouble doing the same. Just my 2 cents.