An "EXTRA BIG RED LIGHT" will be more usefull. You should be spending more time looking at the road than the guages
. Your most likly to get low oil preasure during hard cornering and your not going to be lookign at the guage, wherase a BIG light flickering will give you an idea if the pickup is being starved. If there's a sudden catostrophic failuer of oip preasure a guage is not going to help, and a general low preasure will show at Idle as a flickering lamp.
If you save your pennies now, you can go for a fully electronic dash with all the monitoring built in. Which will cost less than a whole array of guages and keep the weight down ( ok it's not much )
If you do go foe an oil guage you need to choose between electric or capiliary. Electric is less fraught with the danger of the capiliary tube breakign and can be easier to install, but the dials usualy have only a small pointer/ dial . Capiliary guagesr usualy have a full dial right round the guage.