I do quite a bit of slitting in 1018 steel, but never with that thin a saw.
I use a range of saws and keyseat cutters depending on the part.
For something like this I would walk away from using a slitting saw, as the arbor/shank and saw are separate pieces, just introduces unnecessary error. I try whenever possible to use a saw(If I can get away with it) with an integral shank, like a narrow width keyseat cutter.
In this case, I would have started with an off the shelf keyseat cutter with the smallest diameter possible. Being that i'm not aware of one .006" thick off the shelf, I would just ground it thinner far enough into the saw to make the cut.
With HSS in 1018, I usually start with 200sfm and a .0005" chipload, and see what it can handle. Being that i've never slit with a saw that thin, you might want to dial that back to start. We slit down to .060" thick saw regularly, but a much deeper cuts.
Thin Slitting saws are much more sensitive to cutting conditions then say endmills. A couple important factors are chip evacuation, flood coolant(I wouldn't mist a HSS saw in steel), saw rigidity(Smallest diameter possible), part needs to be rigid, saws don't perform well with vibration. etc.
IMO, when it comes to slitting saws and such, using the smallest diameter saw is my starting point. Using a 3" saw to cut .040" depth seems like the first thing I would change.