I recently changed the oil and filter. I considered going fully synthetic, but various searches had that as a no no. After a bit of shopping around, I found this.
http://www.eurocarparts.com/ecp/p/c.../engineoils/4-stroke-oil/?521772072&0&cc5_864 Although, I think it was a bit cheaper then.
All went well. started her up, and the valve train rattle went away after a few seconds (just like normal). Took her for a spin, and all was good. The next day, I went for breakfast with a mate to the 'The Ace', and as she got a bit hotter in traffic, I noticed the change in to 6th was getting notchy.
The next proper ride I went for was the same. As the bike got hot, the gearchange deteriorated, and I'd often get a false neutral between 5th and 6th. So, I changed the oil again, going back to the Castrol Power 1 that North Oxford use. After that, everything returned to normal.
Based on my experience, the 15w/50 might be better.
As an aside. Whilst investigating the notchy gearchange, I noticed that I'd worn away the bottom of the gearchange rubber with my boots. The new rubber was the best £3 I've spent on the bike. I might have got away with the Valvoline!