The above is correct; just to add some information:
For a forward converter, the transformer should have high permeability. This rules out #26 anyway. (#26 would possibly be usable for the accompanying filter choke---you do have a filter choke, right?)
I've used W material before, at up to 0.3T at lower frequencies (20kHz), and didn't get excessive heating. At 80kHz I would expect 0.1T is still adequate.
The W material shows mu'' dominating over 100kHz, which if the mu(freq) curve still holds at large signal levels, suggests the magnetizing current is mostly power loss. But because |mu| is still quite high (~10k), magnetizing current is low, and so is power dissipation.
The situation will be better for J material, the lower mu having a higher cutoff frequency, and so allowing somewhat higher flux density for the same total core loss. But keeping it to 0.1T should still be very safe.
For a flyback, or for the forward's filter choke, neither material is very good. A solid ferrite toroid is a non-starter: the mu must be low, because the core must store energy. Energy storage is inversely proportional to mu.
#26 also has a frequency cutoff around 50kHz, so it's basically all loss at higher frequencies. It can still be used there, but the ripple fraction must be kept very low so that there's more DC output than AC switching ripple. Same as with the transformer---the (AC) magnetizing current is basically all loss so you need to use lots of turns (lots of inductance, giving a low ripple fraction, and low delta B) to keep loss down.
Preferably, use better core material, like #8, or a more expensive mix like Sendust/Kool-Mu, or a ferrite with air gap.
Tim