First off it ain't the oil.
I've run Mobil-1 4T 10W40 in all my bikes for years with no problem whatsoever.
What's the master cylinder fluid level?
If it's too full, there may not enough space for the fluid to expand, causing pressure to build up in the slave cylinder.
For the SP2 clutch, did you make sure you ordered the special innermost friction plate with a larger I.D. that fits around the judder spring and seat?
And did you install the outermost plate in the shallow grooves and not in the full length grooves the rest of the plates fit into?