OK people, another update for you!
Firstly, thank you to everyone for your invaluable advice! It's good to have a wealth of information all in the one place to reference when needed.
So here's what I did; I drained out all the oil and replaced it with Honda GN4 10w-30, which is the recommended oil in the manual. Took it for a ride and although it felt a little better but the slipping was still there. SO I left it overnight for the oil to settle.
Today I rode back from Wollongong to Nowra (70-80kms). It became apparent that the slipping was not only still there, it was getting worse. Riding along at 80km/h and reaching the 100km/h section, it took at least two minutes for the bike to slowly creep up to 100 through very delicate throttle movements. I ended up going through some smaller towns in order to avoid large hills, but I always seemed to end up being stuck behind someone who didn't quite understand what the accelerator pedal was for!
So after a horrific ride home, it now seemed that it was definitely the clutch gone. I wouldn't have made that deduction if there had been even a marginal improvement in the clutch, but as it got worse I sadly realised that it was indeed the clutch plates. I rode it to a mechanic in Nowra and told them the quote the other shop had said. They were stunned (as I believe a lot of you were, as was I), and they looked it up on their computer and told me the engine didn't need to come out and it was only estimated at around 4 1/2 hours. So with that and the parts, it's still well under $1000, so now it's there awaiting repair. Still expensive, sure, but it really does feel like the clutch is well and truly going. By the end, it was bordering on unrideable, and I don't think a number of oil changes would ultimately have returned it back to a fully functioning clutch. I'm sure the "test" the other technician did on it didn't help the clutch situation any, in fact, I think it made it worse. Definitely won't be going back to them. That bridge is burnt.
So now I wait for the final bill, but sometimes these things happen, and new shop guys were really nice. They were almost apologetic over the new price, but I was fine with that.
I just want to get back to riding again. :/