So googling what viscosity to use in my brother in law's EL falcon I see a thread stickied on the front page about oil viscosity....

I don't agree with some of the key points that the thread is based on. First off, when something is called 15W40 or 5W40 etc; it's just a grade. Not an actual weight. We need to get the "weight" idea out of our minds. Elaborating further on this - the grade is much like a grade in school, to put it simply. If you received an "A" you must have scored between 85-100 percent. Same goes for viscosity. One 15W40 will be different in actual kinematic viscosity to another 15W40. Some may scrape into the 40 grade tolerance and are almost a thick 30 grade. Some are right on the edge of becoming a 50 grade. Just like one person scoring 71 on a test and receiving a B and someone else scoring 83 and getting a B also.

Make sense? So that blanket statement of 0W40 - 5W40 - 10W40- 15W40 all being exactly the same at operating temp is incorrect.

What is also wrong is the statement about the W rating meaning it's a SAE5 when cold etc. 5W20 and 5W30 will always be thinner in winter conditions than 5W50. As I said, it's a GRADE not a weight.

To keep it simple, here's an example of two oils. One is a 5W40, one is a 15W40. I have both of these oils in my shed right now and I also have the data sheets downloaded from their respective web sites.

LSA Diesel Ultra 15W40
Shell Helix 5W40

LSA has a cSt of 107 @ 40C and 14.5 @ 100C
Shell has a cSt of 79.1 @ 40C and 13.1 @ 100C

Both are 40 grades as you can see but the 5W40 shell is thinner at both cold temps and hot. Also, 40 degrees celsius is not exactly a cold start. In truly cold weather those numbers will skyrocket upwards. Oil is thicker when cold and thinner when hot. The shell is the superior choice in a cold start situation.

Compare this to a 5W20 as well. The 5W20 Penrite enviro oil is:
49.8 @ 40C and 8.6 @ 100

So 0W20 and 5W20 oils are FAR thinner on startup than any 40 grade oil can possibly be.

With that said. There are some confusing oils. Like Castrol Edge 10W30. Here in Australia, that oil is thinner in all accounts than the 5W30 Castrol Edge. This is because the 5W30 is aimed at a higher class ACEA (Euro) specs and approvals. If it falls between certain parameters they can call it what they want on the bottle. Limits of 30 grade are 12.51 @ 100C if I recall correctly. Castrol Edge is 5W30 is 12.0 cSt @ 100C and 71.8 @ 40C (close to a "thin" 40 grade). The 10W30 is 10.47 @ 100C and 66.2 @ 40C. BUT the 10W30 is aimed at ISLAC GF5 specs "resource conserving" specs. Eg. Fuel economy....

Conclusion - It probably doesn't make a massive difference but in something like a new turbo falcon, I would caution on using some 30 grade oils. I know the penrite 5W30 full synthetic is so close to becoming a 20 grade. That oil is 9.6 cSt @ 100C and 52 cSt @ 40C. The high powered turbo engine may/will shear that down especially if it has and modifications to power output. I'd be using the Euro spec Castrol Edge in that if I had to use a 30 grade.

So research the specs of your oil first. Learn some more about what all the numbers and approvals on the back of the bottle mean. Buy an oil temp gauge - if your high powered engine is getting over 100 degrees celsius, you should use a thicker grade. Especially if racing. I know plenty of turbo and S/C engine can get well over 100 degrees on a hot day with air con on and giving it a blast down the highway.