I ride a 1996, 900RR. And my clutch started prematurely slipping. SAVE your old clutch plates and they will come in handy.
In my case the clutch was slipping because of the full synthetic oil. The friction plates still had alot of carbon. A buddy/mechanic of mine gave me a spare friction plate (the Last one or smallest), and we just added it on top of the last friction plate. Works like a charm. Of course it is just temporary, and you have to baby your bike until you can get a knew clutch. Keep in mind my plates, still had plenty of carbon. I don't recommend this procedure (temporary fix), if your friction plates are gone. To much heat can cause warpage to your steals.
My buddy sais I can ride until it starts sliping again, But I think, it best to get a knew clutch...He's a bit of a miser.

 

Wait........I'm confused. You put in full synth oil and it caused the clutch to slip? Not saying it can't happen, but it has never happened to me. What brand of oil did you use? Was it car or motorcycle oil?

 

The wrong oil can cost you a clutch. It usually the additives - friction reducers. Buy motorcycle specific oil and you will never repeat the problem.

 

The "full synthetic" oil isn't what made your clutch slip, just so you know.
The only real difference between synthetic and regular oil is that synthetics have less impurities and the molecules that make them up are more uniform in size/structure. This allows for better thermal stability, flow, viscosity, etc.
If anything, synthetic is better because of the aforementioned properties. It's used as OEM preferred oil in most wet clutch bikes on the market today.
I hear the whole "it was the synthetic oil making my clutch slip" argument quite often and I'm not sure why.
I (and many others) use synthetics almost exclusively, but if that's not enough proof, we have a member here who has run his F4i to the 200,000 mile mark on the ORIGINAL clutch using nothing but synthetic oil.

As for the extra friction plate, you're right that it's not ideal but will get you through in a pinch. Heavier springs will likely do the same, and so can scuffing your steels.
You may want to consider new/heavier springs when you replace the plates, and make sure the steels aren't burnt or warped.

 

I pulled my clutch apart in my 87 Hurricane and measured all the tolerances. the only thing out of spec was the springs...2mm too short after 23 years and 65000km. Replacements are on order.
Honda builds a tough clutch.

 

I know this thread is more about a temporary fix than another oil thread, but as said, a synthetic oil itself is not what caused your clutch to slip. If it was automotive oil with energy conserving additives, then it probably was the cause, but not the fact it was sythetic. I've ran full synthetic for over 20,000 miles in 1000RR and have never had an issue. Granted, I'm not a super aggressive rider, but I am more aggressive than most.

How long was the oil in the bike?
Do you clutch or power up wheelies?

Its nice that you shared the information. But in all honesty, I would just order another clutch. Unless its your only transportation and you absolutely can't be without the bike, not much sense in temporary fixes to keep riding the bike. Might as well fix it the right way the first time.

 

Sorry. I did not intend to jack the thread, making it another oil thread. That part just stood out. In a pinch, yeah the extra plate will probably help for a while. But you'd not want to ride to long like that, then you may be replacing more than just plates, you'd be doing friction, driven, and springs.

 

Yeah, that was my thought, too. If you have an extra friction plate kicking around, then where is the rest of the set? lol.
And once you're to the point of adding a plate, why not just add them all. I mean, 98% of the work is done... why do it twice. Not that it's terribly hard, but I'm a lazy ***.

 

i agree, if yer gonna split the case...new clutch