I have a 2000 XJ. The oil pressure is fine when running, but when I pull up to stop lights or if i just stop it drops really low. Any ideas on why it does this?

 

oh and let me mention that this all stuff after putting a synthetic oil in

 

It's a proven fact that synthetic oil flows better than conventional, in your case it's just not building up the same pressure as before, which isn't a bad thing. Remember oil pumps create flow and bearing tolerances create pressure..since the synthetic can flow better into tighter tolerances its not gonna create the same pressure..in other words you are fine. What weight oil did you choose might I ask?

 

I took it to valvoline they used maxlife full syn. Im pretty sure they used 5W30

 

As long as there's some kind of pressure, low at idle isn't too awful.

It brings up an interesting question, though... and forgive me if it's already been addressed elsewhere, because I've not seen it, but... since the stock XJ specs call for 10W-30, would it be beneficial, when using synthetic, to use 10W-40 or 15W-40 or something? Would that increase oil pressure?

I'm using the Rotella T5 blend, at 10W-30, but I only have an idiot light so I have no idea what the oil pressure is.

 

You could try changing your oil filter, see if that brings the pressure back up at idle.

 

whats the reason for using synthetic ?
jeep likes cheap stuff i heard that it well leak more if you use synthetic

 

myth. they have the same seal conditioners as regular oil.

the only time i've seen this is when the oil level in our shop truck was low and starving the pump under hard braking...but i'm not gonna ask if you have checked your oil level. do your revs take a dive under hard braking?

 

10psi of oil pressure for every 1000rpm is the rule of thumb, 13psi is the lowest it should ever be on a 4.0.

 

Would running a heavier weight oil help him out?

 

no hard breaking or anything. It does it once the vehicle gets up to the normal running temp

 

What kind of oil filter do you have?

 

Don't mess with oil weights. W30 and W40 grades only impact PSI a small amount in a healthy 4.0. Synthetic 5W30 and conventional 5W30 are both 5W30.

OP, what exactly is your oil pressure? To someone with a busted sending unit 40 PSI might seem "really low". Check with a mechanical gauge.

As noted the minimum spec is 13 PSI. Most healthy 4.0s seem to run around 30-40 PSI at HOT idle. This means the oil temperature is around 210 degrees - takes a while for the oil to get that hot after the coolant does. Multigrade oils and varying ambient temperatures make cold startup measurements pretty much worthless, so pressure can only really be compared when the oil is known to be that hot. (Exception of course being a 5W multigrade oil should cause less pressure than a 20W when cold)