Viscosity is temperature related on a logarithmic scale. Here are the viscosities for a couple of Mobil 1 oils that I got a long time ago while working on engines.
Temp------5w-30------15w-50
-20----------6284--------21,852
0------------2129---------6462
25------------697---------1836
32------------531---------1348
50------------281----------657
100------------71----------138
150------------27-----------46
175------------18-----------30
200------------13-----------20
225------------10-----------15
250-------------7.6---------11.1
A 5w-20 oil would be similar at lower temps but thinner at higher temps than the 5w-30 oil.
As you can see, the 15w-50 oil would be a bad choice at cold temps, but fine at temps of 30-50 degrees and warmer. And likewise, a 20 weight oil at operating temps (probably single digit viscosity) won't provide the same bearing lubrication film strength that the other oils would (think climbing a mountain, pulling a toad, in hot weather, in a MH with an old cooling system).
Here's another chart (but the headings don't line up correctly).
SAE J300 Viscosity Classification - Motor Oil (January 2009)
SAE Viscosity Grade Low Temp. Cranking
(mPa.s)
max at temp °C Low Temp. Pumping
(mPa.s)
max at temp °C Minimum Kinematic
(mm2/s)
at 100°C Maximum Kinematic
(mm2/s)
at 100°C Hi-Temp. Hi-Shear
(mPa.s)
at 150°C @ 10/s
0W 6,200 @ -35 60,000 @ -40 3.8
5W 6,600 @ -30 60,000 @ -35 3.8
10W 7,000 @ -25 60,000 @ -30 4.1
15W 7,000 @ -20 60,000 @ -25 5.6
20W 9,500 @ -15 60,000 @ -20 5.6
25W 13,000 @ -10 60,000 @ -15 9.3
20 5.6 9.3 2.6
30 9.3 12.5 2.9
40 12.5 16.3 2.9
40 12.5 16.3 3.7
50 16.3 21.9 3.7
60 21.9 26.1 3.7
5W-50 6,250 @ -35 30,000 @ -40 16.9 18.0 5.0
Go to the link below to make sense out of the columns above:
http://www.lube4life.com/viscosit.htm#SAE_J300