am i correct in thinking that when i fit my oil pressure gauge (elec) i lose my oe one sender unit if so does this do any harm

 

Does'nt do any harm, its just that you wont have the warning light on the dash anymore, wich just so happens isnt worth a w@nk anyway because it only comes on at a stupidly low pressure. Something like 8 psi IIRC, by wich time the engine be well and truly knacked anyway and so not worth having IMO!