I changed mine a couple of weeks ago and decided to go with 12.5 w oil, ( bottle of 10w plus a bottle of 15w, mix thoroughly ) not as soft and quisy as 10w and not quite so harsh as 15w, this not a rcommendation, its just what I did and so far it seems a reasonable compromise . supporting the front end is actually not that simple if you don't have some proper gear, I pushed long 1/2" socket set extensions into the front engine mount tube, one from each side and then supported them on axle stands,
So finally go ground to the oil and seal replacement, relatively straight forward a couple of minor apprentice type mistakes But what a difference the 15 w oil makes, didn't think there would be that much difference, much more stable under braking.This is a good upgrade only thing is the back end feels soft now!
Oil wise i reckon both, but its marvellous.Rear shock is orginal, in good condition with 24,000 miles on it. Preload is one notch into hard.Work mate suggested a place near us called Protech i think, take in the old shock, for 160 quidish they give you a new multi adjustable one with original end fittings to bolt straight back in. They will get the right spring weight as well. He rates them as he uses them on his drag bikes as well.I feel an all nighter at work coming on to pay for that.
Oil refilled. Forks assembled and 50 miles later the summary is: What a difference!!! I've just joined PRAISE 15W OIL CLUB!
It probably is a combination of very tired old thinner oil replaced with brand new thicker oil, but yes it does make a great difference in my opinion.