Would you please explain this. Are you talking synthetic oils or oil additives like STP?
I based this on the fact that every motorcycle and ATV I've ever owned had a wet clutch AND stated in the owners manual to NOT USE Energy Conserving oils. This is because the Energy Conserving oils have a friction modifier that causes slippage in the wet clutch.
I stole the below statements from here, it explains pretty good:
Using Non-friction Oil with Wet Clutch - ThumperTalk
Very early on, when levels of some oil additives were being lowered, some alternative additives were used as a replacement. One of them at the time was molybdenum disulfide(MoS2). This element is used in gear oils and greases, and at that time also in a couple of new motor oils. This element will indeed cause problems with a wet clutch situation. And it was found out real fast that this was the case.
I think the issue was initially tackled by the Honda folks and the reports of their Goldwing bikes having a massive slippage problem from using the recommended Honda shelf oil. They found out that it was indeed the MoS2 that was the culprit.
Fastforward to today...
It didn't take long for oil makers to realize that another element was needed to replace the slip causing MoS2 additive. The oil makers, and their multi-million dollar tech staffs, realized that using molybdenum diathiocarbamate (MoDTC) was a much better alternative. It was not only oil soluable, unlike it's solid predeccesor, but was proven to not be a problem in the wet clutch situation.
And the wet clutch was only one minute part of the engineers considerations...as MoS2 is a very antagonistic element, which can actually create other unwanted chemicals when mixed with other chemicals and elements. Specifically, MoS2, when in the presence of other chemicals, can create a substance known as sulfuric acid, which is not a good thing in a IC engine that utilizes elastomer seals.
But...it was too late...the myths and legends were already started...
And continue to this day...and mostly passed around by thge less-than-learned, and those wanting to seem as if they have some sort of knowledge in this area.
The truth is about 180degs out of phase with what these folks will tell you. They simply lack credible information...and are backing their warnings up with myth, legend, and hearsay.
Sure, some MC mfgs will state to not use Energy Conserving oils, as they were the ones that would be using alternative elements...and some of these oil makers may simply still be using MoS2...and the bike mfg is simply convering their ***. They are not making a definitive statement by their recommendations. Again, it's just CYA, and nothing more.
Besides, EC (energy conserving) oils do not come in the viscosities that are recommended for use in motorcycles. It shouldn't really even be an issue...but as we see on a daily basis, the issue gets perpetuated by the less-than-learned.