I recently noticed my oil pressure gauge fluctuating while driving and idling on my 99 Cherokee 4.0. It used to stay right at 40 while driving and when idling it would drop a little. Wondering if the sending unit is bad or something else.

 

That would be the first suspect.

 

You're oil pressure changes when ya idle and ya drive so what's the problem?

 

How much does it fluctuate? A little is normal. If it's a real concern to you, You can verify your pressure with a mechanical pressure gauge connected to where the oil pressure sending unit is connected. While you are doing that make sure that the sending unit it's self isn't getting plugged up with sledge.