Till now I've been using Castrol Power 1 GPS 4T 10w-40 semi synthetic (green tub), been to halfords today and they don't have any.
Apparently it's discontinued, replaced by Castrol Power 1 4T 10W-40 Part Synthetic (gold tub) which they don't have either!

They have it in 10w30 but not 10w-40.

I'm sure with a bit of digging around I can probably find the replacement but I want it to work, I don't want to put it in and have clutch issues.

What do you use?

(I'm running a 5VY R1)

silkolene comp 4, for my BEC and road bike

I used to use the red Castrol, smelled lovely.

But then I changed to the halfords own brand semi synth, due to the fact that my engine (s) seems to operate a total loss lubrication system!

I also did use gps and now use its replacement power 1. If you search on ebay for "castrol gps" there's a seller called bike busters who does 4 litres and an oil filter for just under 30quid inc post.

had putoline sport 4 in mine

halfords own brand semi synth for me, change it after every track day, filter every other.

Local bike shop has putoline sport 4, I was very tempted to buy it today, chappy says hes used it in quite a few bec's both kits and grass trackers and never had any probs. He says it's one of the best for clutch's.
Need to change it at the weekend so mail order might not be quick enough.
I'm passing a different halfords tomorrow, i'll see what they have. If they don't have owt I might give the putoline a shot.
I've already got a pile of filters as I one a £50 voucher for Colin Appleyards.

I use silkolene comp 4 in my 5vy engine. Been changing it once a year which has been around every 1000 miles

Rock Oil Synthesis 4 Racing 15W50 is what goes in my turbo ZX10R engine.

http://www.bigccracing.com/shop/category.asp?idBrand=77&showall=true

My local Suzuki dealer recomended Morris sport 10w40 semi syn for my busa engine. Can't really comment on what it's like though.

I'm on Silkoline comp4 in my 5VY R1. I was on Valvoline 4T fully syn' for the last 3 years.

THE most important thing to look for when you buy oil for your BEC is the JASO MA or MA2 label on the can. This is a test to indicate that the engine oil will NOT affect a wet clutch. Many oils are designed for engines with a separate dry clutch so the oil can have additives in it that will improve performance or fuel consumption. BUT a lot of oils like this will cause clutch slip or drag as reported in this thread.
Do not be tempted to buy fully synthetic oils thinking that this will benefit your engine. Also stick with 10W 40 not 15W 50. Thick oil takes longer to circulate and contrary to belief does not increase oil pressure. Most bike oil pumps are extremely efficient and will pump around 25 litres of oil per minute at 5000 rpm or above. This means that the oil is pumped around the engine 6 or 7 times a minute. Thick oil will not be pumped at such a high rate.

I hope you find this interesting.