2005 YZ 125 Gear oil

According to Yamaha, an SAE 80 gear oil (specifically a 75W-80 from their original recommendation). So a transmission oil in the 80 or 80W range good for wet clutches or an engine oil meeting JASO specifications for wet clutch compatibility in the SAE 30 viscosity range would meet the spec exactly. I'm sure a 10W-40 meeting JASO specs would also be totally fine.

https://cdn.yamaha-motor.eu/owner-manuals/Motorcycles/M1C330_1.PDF

According to the link to the Yamaha manual above, for the transmission they call for Yamalube 4 10w-30 or SAE 10w30 type SE motor oil (Page 68 of the PDF / page 2-1 if you're looking at the bottom of the pages)

Do yourself a favor and get a service manual for your bike. Best money you'll ever spend on it. You'll learn a ton (I love reading service manuals, but I'm a geek like that) and most importantly, your shit will be right.

2 strokes really aren't picky about transmission oil, over the years in different bikes ive used 10w-40, 80-90 gear oil, and ATF, all with no issues at all. Obviously the yamalube that the manual calls for is probably the "best" for the bike. That said my trans oil of choice is ATF because it does everything it needs to do and is a lot cheaper than buying the OE oils that are recommended by the manual.

Amsoil, maxima, rotella 10-w40. Make sure its for a wet clutch or it will slip

Yamaha clutches don’t like synthetic oil,Yamalube makes a transmission oil.

There's something magical about Bel-Ray Gear Saver 80W. I've been using it since 2006 and I'll never go back.