All Honda hydrostatic-drive snowblowers use Honda Hydrostatic Fluid. It's special formula fluid with no aftermarket equivalent. Do no use any other fluid except Honda Hydrostatic Fluid in your Honda snowblower's hydrostatic transmission. Genuine Honda HST fluid is available from any Honda dealer. Here's a link to find one there in Canada:
Find A Honda Dealer (Canada)
Maybe you have an an HS624? Don't think Honda made a HS625. Here's a link to the US-version of the HS624:
http://cdn.powerequipment.honda.com/pe/pdf/manuals/31767721.pdf
The HST fluid is good for the life of the snowblower. It is normal for it to be dark after a few years. As long as it is working fine, and there are no leaks, I would strongly recommend leaving it alone. It's big job to fully drain, fill, bleed etc. the HST in one of these units, and again, not necessary if you are not having any problems.