I once asked a 70 year old Sikh gentleman as to what naam is to him?
He said, follow what's written in the Guru Granth Sahib. Har Naam, Har, Har, Har, Har.
I asked him what about Waheguru ? He said its ok to say other names - whatever is written in Sri Guru Granth Sahib Ji is acceptable as Naam Japna - Like Allah, ishvar, prabh, narayan, etc - otherwise they wouldn't appear in the Sri Guru Granth Sahib Ji.
When I asked him that they could be metaphors, and he said - arguments are all created by mind (manmukhs).
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/gurmat-vichaar/18394-naam-japna.html
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/showthread.php?t=18394
The Gurmukhs - don't question. They follow simple instructions in the Sri Guru Granth Sahib Ji.

Asked how should one Jap Naam?
His answer is - treat every breath as your last and remember God in that intensity. Don't analyse anything because mind will take charge again and keep you in delusion.