Mixing Yamalube with other brands?

Just bought a pint of Yamalube forkoil 10W, to add to my front forks, via draining about 1/2 cup from each fork. I think they could use stiffening for my tires sake. Yamalube's bottle states to only mix with yamalube but assorted weights could be mixed, etc. I think it says to not mix with synthetic oils too. Another was don't mix with contaminated oil. Some people say they use Dextron II ATF so is there a reason on the Yamalube? Says flush the oil to use Yamalube, is there a cheaper way than a motorcycle shop? Anybody answer this? Thanks