I done programming lots of genuine 12F1840 pic's with 12LF1840 settings on pickit2 without any problems.
Lets try it, it works for me.
That's right.

As I have mentioned in my earlier post, the PIC works fine. I understand that there is no difference in programming voltage between 12F1840 and 12LF1840 . I havae a PICkit2 programmer and it is able to program the PIC. So, there is no problem in using those PICs.