JHL, no offence, but that's nuts. In the US atleast, oil is cheap, and you can have it recycled for free. I've had way too many motors apart and seen all the black crud in them from old oil.
ANY internal combustian engine will deposit combustion byproducts into the oil over time, the oil itself may last a long while, but being constantly contaminated is hard on it. Anything that goes from a nice amber color to pitch black is not going to last that long without loosing something, that's about as simple an observation as one can make. Have an oil anylasys run sometime and see what turns up. It's only just recently that late models have started haveing extended oil change intervals, mostly because of either synthetic oil or much improved combustion effecniency. But there your cars, so..
On the oil weight issue.
The two reasons for light weight oil being spec'd for newer cars are 1: they are, as was stated, made to tighter tollerences and run much more effeciently overall than older motors. 2: Especially on small displacement motors, a heavy oil can have a noticable impact on emmesions and millege through parasitic drag, even a V8 will rev more freely with 5w30 than 20w50. That being said, perhaps a fresh and well built older motor would be fine with a light oil, but not most of them, in my experience. You take your normal everyday 100K mile chevy 350, it will rattle and knock slightly and generally just sound bad with a light oil in it, but fill it with 20/50, and a lot of that goes away, along with gaining higher oil preasure. Personally I like a pretty thick oil, I've never had a motor in good repair take more than a second and a half, even in pretty cold weather, to build oil preasure with 20/50. That's with a mechanical gauge. The drag is neglagable when weighed agenst the possible breakdown of lighter oil when under high heat and stress applications. There is also the issue of the viscosity modifiers used in lighter oils like 5 and 0 w 30, there not a really great thing either. This is why most true "race" oils are strait 30 weight or so, they also dont have to deal with many cold starts, which is the only reason for the multi viscosity that I know of.
Evolution1980, the outside air temp is the one refered too. You can find that in the manual for the indevidual vehicle. Now if it's an old vehicle, oil has improved some, so who's to say if it's accurate. For example, I have these handy...
For my 78 Lincoln, 460 4v..
5w30 is good from -20 to 100+ F
10w30 is good down to -18 and up to 100+ F
30w is good down to 4 and up to 100+ F
No mention of 20\50
This thing has 214K miles, it has run 10/30 for the first 100K, and 20/50 since then. Holds 60psi hot at 2Krpm, that drops some and it makes various noises with lighter oil, but it's solid as a rock with 20/50.
For my 81 VW Diesel..
5w30 is not recomended at all
10w30= 0 to 60 F
20w50= 15 to 78 F
Only thing rated for over 80 is 40wt
So technicaly my 15/50 is to light for most places I drive in the summer. My experience has not proven that, 20/50 was fine for a long time in several motors, I'm now running Mobil 1 15/50 as sort of an experiment. And still changeing at 3-4K miles, if you've ever touched used diesel oil you'll understand.
Those are the only specs I have handy, and if they were to write them today they would likely be different, as oil has improved quite a bit. The trick is to not mistake oil claims because of actual oil imporvement, and those made becasue of modern effecient motors. The VW's a good example, the new TDI's can run 10K miles on an oil change with specific synthetic diesel oil, this is proven by oil anylisys, but would I run my trusty old two decade old 1.6 that long? No way.
I'd say it's probably a matter perspective, just like cross-drilled -vs- solid rotors.
You would want to look into a pre-oiler if you're concerned about that. Thicker oil would adhear to the parts better, as opposed to thinner oil running off them faster after the car's been shut down. 
