The 'liners have a wet clutch and the manufacturer suggestion is 20w-40 (based on the owner's manual). However, the currently recommended(by Yamaha) OEM oil is "Yamalube Semi-Synthetic 20w-50" (http://www.starmotorcycles.com/star/yamalube.aspx), and is not offered in a 20w-40.
Like other vehicles, other oils can be used. The key with the wet clutch is look for oils which are NOT "energy conserving". (You will find that any mufti-viscosity oil with a 2nd # of 40 weight or higher will NOT be energy conserving, and is therefore acceptable in wet clutch applications).
Here are some of the other oils used by members of the forum:
- Amsoil V-Twin 20w-50
- Mobil 1 V-Twin 20w-50
- Shell Rotella T
- Lucas High Performance Synthetic SAE 20W/50 Motorcycle Oil
The general consensus is that the semi-synthetic "Yamalube" 20w-50 is the best oil for the Liners. The motor appears to be quieter with this oil, for those who've experienced the lifter noise problem.
There have been some complaints of more engine noise with Full Synthetic (This has not proven true with the majority of Full-Syn users, but for those who've had that experience, it's near 100% certain - you be the judge).
Oil Change/ Level Check
Most Informative Article on Motor Oils
http://www.boss302.com/oil.htm Written back in '03 but still very applicable and points out or debunks some myths like 'thicker is better', 'racing oil is good', 'synthetics are too slippery', etc. Is a little lengthy but technically easier reading than many articles. Very understandable and informative, and definitely worth the read.
More information about oils may be found at http://en.wikipedia.org/wiki/Motorcycle_oil.
as well as here- www.bobistheoilguy.com
And another excellent oil discussion kicked-off with an article written by our own 'Buck' (of Buck's Lowers fame)