I swapped the motor and trans from an 01' cherokee into my scrambler. It works great but from time to time it won't build pressure when I first start it, I usually just kill it and let it set for a moment and it usually builds from there if not repeat once or twice. and always has good pressure once its there. I thought it may be the oil pressure sender changed it and no change...now I have a spare. I'm running 10w-30 oil. Any thoughts? Has anybody else experiance this?

My 91 does this too, and usually a lifter taps for a little bit until it builds up. I put in a quart of Lucas and it didn't really help, it just brought my running oil pressure up a bit.

You need to get a trustworthy mechanical gauge (Autometer) before you get worried - or plumb a decent gauge right off your sender port while starting the engine.

Never, ever trust the factory gauges.

If a good gauge still reads low at startup, you may have to start tearing into the engine.

Try using an oil filter with the "anti-drain back" feature. Purolator Pure One and I think the high end Wix filters have it. Its also a problem with Nissan 4cyl's and using the Purolator Pure One fixed the problem on my ex's Sentra.