I'd be looking for something more external...
Did all the work you did by any chance disturb the cables from the shift/throttle mechanism? Often if the cable clamp at either end comes adrift you end up with an off-axis psuh/pull that can bind and lead to stiffer action. Check that the sleeve clamp at the gearbox end esp is still firmly fixed and in line with the lever. If it's easy to get to check the other end as well.
Another thing you might try is to disconnect the cables and see if they move smoothly off the gear, and see if shifting by hand seems OK or not.
Another thought... all the motor mounts OK? Higher torque may be forcing the motor/gear out of alignment..with the cables (real long shot..)
Being familiar with the Hurth gear it's difficult to imagine an oil change affecting shifting effort. In any event despite Hurth's claim of being able to withstand shifting at high revs I'd not get into that habit..
You did just put a 1/2 liter of oil in, right?.. the 'full' mark is just 1/4 inch or so up the dipstick and should be checked without the threads engaged (sorry, gotta ask...)
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)