I had a '79 Honda 750. I pulled the ATF out of it, and it was a dark, Hunter Green color... No idea how long it had been in there, but the bike had 10,000 miles on the clock, and I was changing it the year after I bought it: 2007! I put ATF in it after I cleaned it out with some
SeaFoam engine oil treatment: dissolves varnish, and gunk very quickly, just add 2 oz., compress the fork tubes 5x to percolate it through the system, then drain, and fill with fresh ATF.
By the way, ATF is around 7.5 viscosity, for fork oil, which is why Honda used it. It maintains its viscosity over the entire operating temperature range -- just like Fork Oils do, only ATF costs roughly one-half of what they charge for Fork Oil. ATF is also highly detergent, in nature. With routine changes, the
SeaFoam treatment should only be necessary once, or you could just change the ATF after a week of riding, as it, too, will scrub out the innards of your fork tubes.
Fork oil is probably the most overlooked maintenance item on most bikes... I change mine every two years, and I use ATF in all my bikes. After two years, it has lost its cherry-red coloring, looking much darker... It's fast, and easy to change out, and it does make for a better, safer ride. YMMV. Cheers!
