Aye Christine,
I use the same oil as yoursel' in Sally, with no problems, even after using other oils [car spec] and only draining and re-filling with a new filter, with no problems.
Using the 'exploded' clutch parts diagram from Steve's 'SRX Homepage' site, you will see that there is a ball bearing in the clutch operational 'chain', between the two pushrods. Have you lost yours? Has yours worn flats on it, rather than being spherical? Do you have the clutch plates and the clutch boss spring in their correct order, as per diagram? If not, it don't work to well! 'VeryNear' calipers required. Is the clutch adjusted as per manual and if that is OK, is the cable adjusted as per manual? Freeplay measured with a tape measure from the bar end weight may seem 'anorakish' but makes a difference.
If none of this throws new light or provides a solution, then after your list of repetative cleaning and assembly, Steve's lovely image drawing a blank, I would start to feel a bit flumuxed!

I hope you find a solution.
My regards, Bill.