Hey, I should add this. Basically oils have a grade number that says how thick (or runny) it is at a certain temperature. I think the SAE oils were tested at 60°F. SAE is the USA Society of Automotive Engineers.
So, a SAE 30 grade oil is runnier than a SAE 40 grade, which would be a little thinner than a SAE 50. SAE 50 would be thicker than SAE 40 or any grade number less.
Now then, long ago in the early days of motoring in VERY COLD temperatures some oils tended to get far too THICK compared to other oils even of the same basic SAE grade, and engines were often so gummed up by thick gooey oil they would not even start. I have experienced that in my early motorbike days. My Triumph 500 GP was a brute to kick-start if SAE 40 oil was in the tank even though it had a "dry sump". The oil was like syrup in cold weather.
In those days there were no so-called "multi-grade" oils, and most cars would be given clean oil every 3,000 miles, using a SAE 20 in winter and a SAE 40 in the summer. These oils were later called MONO grade oils. Chancers used SAE 30 all the time.
The oil companies put their brains in top gear and worked out ways to make oils that stayed thin (like a SAE 20) when cold and stayed quite thick (like a SAE 40) when hot. These oils behaved as if they had multiple grades, so they called them multi-grade oils, and they gave them TWO grade numbers such as SAE 15-40 to indicate it behaves like a SAE 15 when cold, and SAE 40 when hot. The W such as 15W-40 just indicates a slightly enhanced cold weather flowing ability.
So ... why do the User Manuals specify all those grades and BRANDS of oils? Easy - because the oil companies pay them BIG BUCKS to do that! An incredible amount of money is spent on oil advertising. They used to say that Castrol spent more money advertising GTX than they spent making the oil! In those days Castrol didn't tell you the grade and the oil was a pathetic blend almost a monograde. The only clever thing about it was the incredible HYPE. Smart people used Duckhams 20-50, but that didn't last after BP bought Duckhams. Things have changed since hopefully. But remember, advertising costs are factored in to the cost of the oil, so guess who pays for every penny of it at the end of the day.
Buy quality oils, buy in bulk, do not buy "trendy" or "synthetic" oils and use the good value to change the oil and filter every 5-6,000 miles.
Changing the oil (that is, draining it right out) is BY FAR the most effective way of removing all the crap, acids, water, carbon black, metal fragments and so on. Using synthetic oils for twice as long means all that bad stuff stays in the engine much longer, eating away at your bearing rings and pistons all the time.
DIY oil and filter changes are so easy, and so cheap, we should do it often. It costs me under £10 to change the oil and filter in my Disco TD5. That's a tad less than a LR main dealer would charge!
CharlesY
105mph then 
