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Its a 5.9 390HP Crate engine. It came without a windage tray and a low volume oil pump. I'm having low oil pressure issues at idle when warm.

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Absence or presence of a windage tray doesn't affect oil pressure. Maybe at high RPM with a high volume oil pump, and low oil levels, it might slow oil return to the pickup, but definitely not at idle. And almost every builder recommends standard (ie "low") pressure oil pumps in small blocks, even very high horse builds. Like Cab Burge said above.

There is always a chance the oil pump that was installed in it was just bad from the beginning. Such as loose internal clearances in the pump from the factory. Depending on how hard it is to drop the pan, I would try replacing it with a new standard volume pump first.

If that doesn't help it there is more than likely is a problem elsewhere in the engine. Missing or improperly installed plug in the block between the oil filter inlet/outlet passages, lifters not covering the oil galley, etc etc. better to find out where it is escaping from. A high volume pump may help if there is something like worn or loose bearing clearances more commonly seen in high mileage motors, but that is about all they are good for IMHO.