After driveing 10 to 15 miles at highway speeds when I stop and the truck starts to idle at about 800 rpm the oil presure gauge jumps up and down like crazy. If I hit the gas and bring the rpms up to about 1000 rpm it stops jumping. I have changed the oil pressure switch and done an oil change using Amsoil 5w20 High milleage full synthetic. I have seen on other Ranger forum threads that this is a common issue but can't find anything that seems to be a reliable fix for the problem. Any suggetions would be greatly appreciated. Thank you.

 

My guess is that your hot idle oil pressure is around 5 to 6 psi, which is where the idiot light switch that powers the gauge turns off or on. Your 20 weight oil is likely one source of the problem. My ranger calls for 5w30 oil which is thicker at operating temperature than your 20 weight, so the oil will leak through the bearings more slowly building more pressure. If 30 wt. oil does not fix the problem you could go for 40 weight. but I would try 30 wt. first. There are also bottles of oil thickeners on the market that might solve your problem until you can change to a thicker oil.