why does oil pressure drop to zero when at idle

Electrical oil pressure gauges are notoriously inaccurate.It may be a bad sender unit, screwing in a mechanical oil pressure gauge to verify actual oil pressure may be necessary. First of course check the oil level is correct. Minimum oil pressure at 1000 rpm with warm oil should be 6 psi, @ 4000 it should be min. 24psi

It should not go that low . Check your oil level and to be honest I just had a night mare of a time with my neighbors truck. To fix his truck I had to do a main bearings and rebuild it.