Ravenol also makes a legit lf6 fluid along with the lf8 fluid.
I'd be comfortable using ravenol 6, but mercon sp is the best deal for the 6hp.

Also for those who don't want to deal with removing/replacing the pan, a drain and fill (and another one a month or so later) is also good advice.
A few people have cut open the filter in the pan to see how it was not clogged at all. It's just there for big particles, and the magnets on the pan capture tiny particles and there's no problem if it built up. In manual trans, those magnets aren't even serviceable without taking the case apart!