I am about to change a fork seal on my Breva 750. According to the manual the recommended oil is 5W with an option of 20W. Talk about both ends of the scale. 
I plan on changing the oil in both legs whilst I am at it, My gut feeling is to go for 10W but I thought that I would see what the consensus is. BTW the forks have progressive springs.
Why are you changing it?
If you feel it is too soft go for the harder option, if reverse then the opposite.
Mine is still the original after 50,000 miles and is good enough for me.
I am obviously interested in how you get on.
I had mine changed last year when I had a major service by Tony Botto
he used 5W
it restored the smooth fork action and made the bike nicer to ride
Thanks gents
I haven’t actually ridden it yet. The seal on one of the legs is leaking. I think it happened when I trailered it home, so I am going to change it. The bike has been stood for a couple of years So I thought that I might as well change the oil in both legs while I am at it.
Obviously I will be checking the stantions for pitting too. I used 5w in my Ducati Monster forks when I fitted progressive springs and the forks were a bit too soft. I will try 10w and see how it goes.
I got around to changing the fork seal yesterday. I had a bit of fun as the mudguard bolts were seized solid and it looks as though someone has had a go before as the allen bolts were chewed. 
I ended up removing the forks as a pair.
Its just as well I was changing the fork oil, looking at the black pissy stuff that came out. I changed the seal on the leaky side and cleaned up the parts, rubbing down a couple of small pits on the stantion that had probably caused the seal failure. I refilled the fork leg with the recommended 400 mil of 10W oil and measured the air gap, with no spring and with the leg compressed. The gap measured 16 cms. I reassembled the leg and removed the spring from the other leg, drained the oil and flushed it out with paraffin leaving it to drain. I then added 400 mil of fresh oil. I used a cheap fork level tool I bought a while back on ebay, set to 16 cms. I drew out about 2 mils of surplus oil.
After putting everything back together, I got my first ride on the Breva. The forks feel nicely damped and the ride was supple, so I am happy with my decision to go with the slightly heavier grade oil. I might have a fiddle with the rear Ikon shocks, maybe firming them up a notch. A pair of seals cost £7.99 inc post (ebay) and the oil £10 for a litre so not too dear.
Overall the bike rode well. My only concern is that the gearbox sounds a little winey. It might need fresh oil. The Breva has been stood for 2 and a half years so it may take a few miles to get everything bedded back in. The engine is lovely and the bike seems very roomy and comfortable. Nicer than the V7 stone I once rode. I am looking forwards to getting some miles under the wheels now. 
I had the same issue with the Mudguard bolts when I did the oil in my Breva forks and solved it in the same way. I also went with the SAE10 oil but not before trying SAE5 and experiencing the interesting effects of under damped forks.
mine whines like a bloody banshee
its not too bad in 5th, although the gearbox had a session with no oil at one point
ps you should really have changed both seals as a pair, they work together and will have done the same work
easier than changing it later
I fitted new rear shocks to mine and then had Tony Botto service it including re greasing the head bearings and swing arm bearings etc fork oil and gear oil etc
it made the bike much more sprightly to ride and is as nice as when I first rode it away from the showroom all those years ago
Russ, is the whine from the gearbox or bevel drive/
Mine turned out to be the bevel box and Nigel at NBS sorted that out for £135.00.
Will you be at the red lion on Sunday?