Engine oil - you could ask 100 people and get 100 answers, but here's mine
If your bike has less than 18,000 miles, use 10W/40 grade mineral oil, eg Castrol GP.
Once you pass that mileage, use either 15W/50 (or 10W/50 if you prefer) grade semi synthetic. I use Silkolene.
Strictly speaking, a 15w/50 should cover pretty much any temperatures you would expect in the UK, but while the engine is still running in, the slightly thinner 10w/40 won't hurt. But it will get burnt quicker.