First thing I would do is check into using synthetic.I read somewhere it is not good for some bikes because of the clutches.As dumb as this sounds it's too slippery.

Suggest first try using MC oil instead of auto oil.  It has additives for the clutch (sulfides?).    

If you want synthetic,  Wallymart , Mobil 1 V-Twin $8.63 / qt.   Not sure if the "V-Twin" label actually give you any more hi temp capibilites like it says.  

I have been reading as much as I can find lately and it seems that many car oils have "energy saving friction modifiers" mixed in that can wreak havoc with wet clutches.  That seems to be THE reason it is not wise to use car oil in motorcycles. I changed my oil two or three times - just to be sure it wasn't oil related.  It's worth trying before you tear into it.  Although, with the price of some motorcycle oils, I am not sure its cheaper.

You will note the casting the clutch springs sit in has milled areas for each spring. Close inspection will reveal they are different depths. The casting isn't the same thickness throughout & they mill each foting for the clutch springs so each spring is under the same load. I measured the thickness of each washer & ground them to fit inside the milled area, so as to not have uneven pressures across the clutchpack. Only used approximayely 1/8 thick washers & no more of that little OOOZE feeling when I am slamming thru gears.
I am running Shell Rotella _T oil, It is just under $9.00 a gallon at Walmart. (Wrll, till oil hits $75.00 a barrel, then it'll go up I guess)

I am betting it's the oil, tho. NOt too likely it's the clutch, IMO.

EDIT* What I meant was, the clutch isnt shot at 16K miles. The mods didnt create a torque monster that cant be contained by the clutch , certainly not with a 50 MPH roll on.  Thats the same clutch you had before the mods. IF it will slip at a 50 MPH roll on NOW, it SHoulda been slipping at 10 MPH in first accelerating away from a stop sign BEFore the mods. I believe the oil is the big deal here, & you will likely spot some slipping as you run thru the gears if you run it aggressively. When I did the carb, exhaust & air filter, I started feeling some slight slippage. When I did the cam chain, I did the washers. Problem solved. I rode it for several thousand miles after the mods before I did the clutch.

All the disel /fleet/ auto oils are excellent and are good bang for the buck.
The so called "energy conserving oils" are so thin, that we don't use them any way. Usually they are 0W-20, 5W-20, 5W-30, and may be some 10W-40. If You  stick with 20W-50 or 15W-40, You will never get "slippery" oil.

I let the clutch lever adjuster be pretty sloppy. It disengages fine for stoplights & by letting it be sloppy, hauling it in is easier on the fingers than whenit is adjusted as far out as possible. All it takes when shifting is to ease the pressure on the clutch anyway. That accompanied with the throttle makes the shifting slick & easy.

scored them on fleabay.

p/n mt10-4

fits several bikes