Best oil for smoother shifting

Yea, i've heard of too many clutches that were "questionable" as far as overall condition, slipping after swapping to a full synthetic based lube. If you know you have a clutch in solid working condition, you most likely won't have any issues. It's just my preference.
You're free to have your preferences, of course, but synthetics, per se, are not the problem. Solo-WR400 has it half right, those troubles come from using the wrong oil, but "car" oil isn't necessarily the problem either. Both statements are an over generalization.

The "bad" oils are ECII (API Energy Conserving Group II) oils. These oils contain friction reducers that may cause a wet clutch to slip or behave badly. It also happens that nearly all of them are synthetics and blended for cars, too, ergo, "synthetic car oil is bad for your clutch". Bad logic.

The best oils available for a YZF are nearly all synthetics or petro/synthetic blends. However, none of these are ECII, and most either comply with or are labeled as JASO MA. One of the elements of the MA standard is compatibilty with wet clutches through the establishment of a minimum friction coefficient. Such oils are, then, certified compatible with any wet clutch system in any motorcycle. Car oils not labeled ECII are probably fine in that regard, synthetic or not.

Car, and even truck oils, do have one weakness, however, and that is shear stability. Multigrade oils are thin oils with additives blended in to keep them from thinning with heat as much as they should, so your 10w-40 looks like a 10wt at 70 degrees, and like a 40 at 210. Transmissions physically tear these additives apart, and the oil can become a 10w-20 in an alarmingly short time. The best motorcycle oils, like Amsoil, Mobil 1 Racing 4T, Golden Spectro 4, and others use viscosity improvers made for gear lube. These cost more, but hold up far better, and are well worth the difference.

Nice narrative. That's as I understand it as well. I am currently using the Mobil 1 4T and I have to say, my 06 feels notchy. I don't know as though a different oil would help. Once the basket gets the notches in it, oil is secondary. The feel has more to do with condition of the bike than use of any of the top grade synthetic oils.

Notchy shifting is not a clutch issue, it's a problem with a lack of smoothness in the operation of one or some of the rather complicated series of mechanical pieces that allows your left toe to move gears around in the transmission. whether oil will fix it is a matter of which part is operating roughly, and why.

Yes, I was confused. My only real notchy shifting on my 06 comes when trying to find neutral sometimes which is because my clutch is grabby sometimes because it's got a bunch of notches on the basket and hub. I've seen them in there and they're really nice notches. I even filed them some to try to save cash cause I don't have any. That's due partly to the dirtbike addiction in the first place. I actually have no troubles with the way it operates when I'm riding. Quite smooth actually.

Rotella T

After reading many of the topics about oil, I switched to the Rotella Synthetic and have noticed a small difference in transmission smoothness and zero problems with clutch.

For the most part notchy shifting simply stems from a new gearbox that needs more break in time. Every new Yami I have owned shifted hard for a few months, than progressively smooth out as the shift drum, and forks start to loosen up.

I have also heard of notchy shifting stemming from older "sticky" clutch drive plates (on 2 stroke engines). A lot of times it's at it's worst when the oil is cold, and not warmed up.

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