Slow charging is generally considered better, for various reasons such as generating less heat and stress on the battery, and for more technical reaons which I'm a bit shakey on but have to do with the "interface charge" and distribution of the charge to the electrodes and electrolyte.
I wouldn't think fast charging is 'harmful" per se, but really, if it is charging overnight, I would think a slow charge would be just fine.
And if your battery gets pretty badly discharged during the day, for sure a slow charge would be better. Heavily discharged batteries don't take a fast charge very well--they get a surface charge but not a lot of power to crank or run anything.
Try 2 amps and see how it works during the day. If the battery is sluggish, then go to 6. I'd avoid continuous 40 amp recharges.
Also make sure you have proper venting because of the hydrogen gas.