If you are going to use an line you may also have to use a gauge snubber as well to dampen the pressure spikes. Those 1/8 copper lines really help dampen the pulsations to the gauge. Almost any an line is going to increase the diameter and the volume of oil that reaches the gauge which will really let you see what your pressure is doing because the extra volume will increase the gauge response.
Having said that I would use the smallest size an line you can get which I would think is 3an. Auto meter also sells snubbers to dampen the gauge spikes to the gauge. You can buy adjustable ones too.
