Yes I know another motor oil post. Just more or less and observation. I changed the oil in my Cat to the Jaguar recommended 5-30 weight. As I was pouring it in I couldn't help but think how thin it was. After adding the 8-8.5 quarts I wondered if this was the right decision. Here's a little background on why I was thinking like this. I've owned several cars, trucks and muscle cars. I have always used 10W-40 oil. Several 68-70 Mercury Cougars all have used 10W-40 oil with no issues with engines. I had a 2000 Taurus with 265,000 same oil with no issues except for the car that T boned it. Dodge Dakota with 248,000 same oil again no issues. 2006 Corolla with 303,400 same 10w-40 still going strong burns no oil. I have always changed the oil at the 3000-3500 mile range. All of these cars ran/run just like there're new. Is changing the oil that often overkill? Should I switch to synthetic? The answer from most would probably be yes to both. But the above stated reasons are why the oil weight bothers me so much. I will probably run this oil for a couple thousand miles then switch to my usual 10W-40. I am in SoCal and the weather is warmer here year round. Especially so this year. Just wanted my thoughts on oil out there. Thanks for letting me ramble.

 

5w30 is more than adequate. I always used 0w40 in my GTO and since I have plenty of it around, it'll go in both Jags next time around.

 

Pour characteristics at room temps don't have much in common with those seen at normal engine running temps. Not much of a reliable criteria in choosing the proper viscosity.

 

I use 10W40 and like the oil pressure readings (RealGuage), but the main reason is because I have a slight rear main seal leak. I use Mobil 1 but there is really no need to use synthetic, especially as often as you change it.

 

With a sump capacity of 8+ quarts, a 3000 mile oil change interval might be overkill given that contamination and breakdown are spread across a larger volume. Something like 5k might be good if only because it fits nicely into the odometer. Trying to track 6K would be just too much bother. And with 10K I would be wondering whether the oil filter has gone into bypass.

I like Shell Rotella 15W40, but there is no VVT to worry about on a XJR.

Viscosity at room temperature is not the same as viscosity at running temperatures, but there *is* a relationship since there is a viscosity curve. There is a site run by a well known oil consultant where you can input the cold and hot viscosities of several oils to graph the resulting viscosity curve. All based on proven and accepted science.

 

DOHC engines tend to need a thin oil on startup to assure good, quick-acting flow in the small oil passages that feed the cams and their related systems, especially those with variable cam timing systems, as those systems are very sensitive to oil viscosity. That's why most modern cars with VVT have moved to 5W or even 0W cold viscosity oils. On the non-VVT supercharged cars, 10w-30 is fine, although I use 5w-30.