You guys do understand what the numbers in the oil gradings mean, right?
In case you don't, here's a quickie crash course.
The first number is the cold viscosity. Dunno what temp this is rated at, but I think it's either 32* F or 0* F. The second number is the hot viscosity, and again I don't recall at what temperature it's measured, but just think of it as "operating temperature," and the first number as "cold start temperature."
All the multi-grade oils, petroleum-based and synthetic, are modified to achieve their multi-grade properties. When I was a lad, back shortly after the end of the Civil War, there was no such thing as multi-grade oils. We ran 20 weight in winter, and 30 or 40 weight in summer. When an engine got really old and loose, we'd run 50 weight in summer.
The viscosity ratings are based on a scale that applies to conventional, straight-weight oils. When you see a multi-grade oil that's rated 10W30, what that says is when cold the viscosity is the same as a straight weight 10 (cold), but when hot the viscosity is the same as a straight weight 30 (hot).
If you have an owner's manual and look at the table of recommended oil weights, there are a couple of multi-grades (typically 10W30 and 10W40, and for the new models 5W30) that can be used all year long for MOST climates in the continental U.S.
Keep in mind that even 50 weight when hot is a lot thinner than even 5 weight when cold. What you're looking to achieve is a balance between cold start circulation (you don't want an oil that's so thick it won't circulate) and hot pressure. For my climate (southern New England) the recommended grades when I bought my '88 new were 10W30 and 10W40. I always ran it on conventional 10W40 until about 175,000 miles, when the oil pressure on long trips began to be lower than I liked (it was still within spec, but lower than I liked). So I switched to 20W50, then I switched to synthetic.
The owner's manual for the 2000 XJ calls for 5W30 in this climate. The reason is to promote quicker circulation on cold starts. In theory, at operating temperature there should be no difference between a 5W30 and a 10W30.
Personally, I don't completely trust that theory. On the older vehicles I switched to Mobil-1 15W50 to get the better viscosity at operating temperatures. The plan was to run 10W30 in winter, but it seemed I was always off cycle and running the winter oil into summer, or the summer oil into winter. Then I discovered Castrol Syntec had a 5W50 synthetic. That I could run all year, and have both good cold start flow and good hot weather viscosity. That's what I run in the older ones now. The 2000 is about ready to switch over to synthetic, so I'll most likely just run the same oil in all of them.
