Ok so my 96 with around 185k on it has been dropping pressure when hot according to the dash gauge. When AR 210 on the coolant temp the oil pressure at idle is about 10~ and at any other rpm it's about 40 again on the gauge. I haven't checked with a mechanical gauge or an external. But I'm running 10/30 Castrol I believe. It needs to be changed. The previous owner never took care of this thing and I'm trying my best to rectify that. In my 90 I run Mobil 1 10/30 stuff guaranteed to 5000 miles and a factory filter. This one I am running why ever and a molar filter. I thought the oil I have at my job was 5/20 turns out it's Mobil 1 5/40. Has anyone run Mobil 5/40 in their 4.0 and had any issues? I don't care about any other viscosity as I can get this 5/40 free. It's full syn oil too. It's formulated for Mercedes specially by Mobil.

 

Chris JMHO but I would not switch to a full syn now and not a 5w40 right now. When you change this time try a 30w oil and a NAPA Silver filter and change the oil often for a few changes 1500-2000 mi. (I never let my oil go over 3000 mi. till its changed) no matter what the oil is. Oil and filters are cheap see if this doesn't increase your oil pressure at low RPM. These temps are very close to normal as is the pressure drop.


After a couple of changes before winter change to what ever oil you want. This should clean up some filth in the engine and the poor service the PO didn't do. I run Castrol 10w30 in my XJ year around and a NAPA Gold Filter or a WIX.

 

5W-40 would probably be fine, it would certainly bump your oil pressure up some when hot. I agree with Fred that you should do a few short OCI's to clear out some of the grime that probably built up in there. If you suspect sludge you could run some ATF or Marvel Mystery Oil in it for a little right before changing the oil.

 

There wasn't any sludge in there when I dropped the old oil. I was doing some work on it for the po and it was like 3 qts low. The oil is relatively clean now. I have done a few changes. Just wanted to know about the 5w 40

 

I'm running Rotella T6 which is 5w40 in 3 vehicles. All right around 150k miles on the odometers. What did you want to know?

 

No noises how's your oil pressure ?

 

Of course I can't give you precise readings due the inaccuracies of the factory gauges. But here's something that may help. One of the vehicles I switched over is a '96 4.3 Blazer. Last Fridayi had to have the intake gaskets replaced due to a coolant leak. The shop had to replace the oil due to it getting contaminated during the fix and of course changed the filter to. They used Advance Auto 10w30 and their Federation filter. Brought it home and it sat. Yesterday I had to make a surprise 300+ mile trip. Just to make a long story short it's not showing quite the same pressures at idle,low speeds ,or high speeds as the T6/Wix combo. No big deal but I'll change the filter to Wix tomorrow and the oil at 1000 or so miles. Sucks when I'm not able to do the work for the time being. But they really do GOOD work.

 

Cool thank you. I'll be doing my change tomorrow