15W Fork Oil. Does it make that much difference?

[Wookee]

I've never had a front end problem that many here have experienced, but I had a leaking fork seal so had to change mine the other week (along with a kebab-supplied R6 shock on the back). The first thing I noticed was that the oil that came out was in excellent condition and very, very thick. Much more so than the 15w I put back in. There is no noticeable difference in the front end since the change and I wondered if for some reason fork oil thickens up as it gets old?? Mine had been in for at least 35k miles.