A slight change to the algorithm fixes it. Multiply the last digit by 1 (not 5), discard "I" and "O" in the alphabet and start at zero when counting through.
This works for all examples in the thread.
Service Number
1941305
1x8=8
9x17=153
4x4=16
1x16=16
3x2=6
0x13=0
5x1=5
A= 204
B= 184
A-B= 20
ABCDEFGHJKLMNPQRSTUVWXYZ (Starting with A=0)
Checkdigit W