You should be able to use any chain/cable fluid. I use whatever's in my box at the time (i.e honda chain/cable lube, maxima chain cable). You can probably use WD-40? I personally like any lubricant that dries (i.e. chain wax) because lubricants that stay wet, such as oils, tend to attract and hold dirt over time, whereas dirt won't stick to the waxes. Spray it well and let it run out the other end (disconnect first), this should flush out any dirt. If it's just dirty, this should fix it. Otherwise you may have a frayed cable and will have to replace. Try the cheapest solution first I always say.
They make a small tool to help get the lube in the cable, (here's one version:
http://www.rockymountainatv.com/productDetail.do?prodFamilyId=2999&navTitle=Chemicals&webCatId=23&webTypeId=100&navType=type) so you don't make a big mess. But you can just wrap a cloth around the top of the cable and use the little straw that comes with cans of lube to help force it down the cable. Also helps a lot to spray a little, work the cable back and forth a bunch of times, and keep repeating the process until you get clean fluid coming out of the other end.
Hope this helps, and good luck
