Thought this would be the best place to get some idea as to the correct grade of engine oil for my 2000 WJ with the 4.0 I6 engine as I seem to be getting a different answer from different sources !
Car has only done 76k miles.

 

Chrysler's recommendation in my owner's manual is 5W-30 for all temperature ranges and 10W-30 if the engine never sees below freezing temperatures. I bought mine with 125K on it and have used 10W-30 ever since. It cranks a little slow at -30*F, but other than that I've had no problems. 212K on it now. 40 psi hot idle, 55 at 60 mph. I'm a dinosaur and still feel 5W is like water.

 

I run Rotella T6 in my XJ with a 4.0. Does great. Search the XJ section, tons of guys swear by it.

 

Among the recommendations from a well known UK car accessory supplier are :
Mobil Super 2000 X1 10w-40 & Castrol Magnatec 5w-30
Checked on the Mobil website - recommendation is for Mobil Super 3000 X1 Formula FE 5w-30
Checked on the Castrol website - Says no suitable product, check owners manual !

I recall the simpler days when 20w-50 went in everything...

 

weight=owners manual.....brand= eniy,meeny,miny,moe

 

Asking what oil is best is like asking what's the best beer, everybody has a different answer. Chrysler recommends 5W-30 because it creates an adequate oil film to support the bearings, yet also reduces power robbing drag on the engine parts at low temperatures. You can use whatever floats your boat as long as supports the oil film and prevents metal-to-metal contact on the bearing inserts. 90% of bearing wear occurs before oil pressure comes up. The soft lead babbit material is designed to imbed hard particles that might cause scoring of the crankshaft, yet must be supported by oil under pressure or it won't last very long. When engines are overheated to the extreme, the pistons expand until they start rubbing on the cylinder walls, melt, and cause seizing.

For all you desert off-roaders, sand and dust are big destroyers of engine bearings and piston rings. It MUST be kept out of the engine as much as possible. But you probably already knew that.

 

Eventually chose the Mobil Super 3000 X1 Formula FE 5w-30 as I found a brand new unopened can of it sitting on a shelf in my garage which I had forgotten about...

 

The largest contributor to wear when cold is the fact that the operating clearances are not to specification. Condensation and corrosion are another factor.Different metals expand at different rates. Until the engine is thermally stabilized the clearances are off. A cold piston can have clearances four to five times normal operating clearance on one axis. Residual oil that remains on bearings overnight is enough lubrication for a few seconds ( actually quite longer) until oil starts flowing. Remember those snake oil commercials where they drain the oil out of an engine and run it (at idle) for 30 minutes? Just a hoax it can run that long without any additive!

 

I've seen videos where they've run engines without an oil pan in them! Without any load and at idle, it's amazing how long they stay together. Of course, they are totally worthless after doing this. In one video, they wound it up until it scattered after around 2 minutes!