The farther apart the 2 numbers (10 & 50) the more VI's (as described by Grayracer) have been added, and therefore are more prone to damage and hence this oil will thin down quicker.
The hot viscosity is 5 times the cold!
Going to a 15W/50 reduces this ratio down to less than 3.5
As you live in South Africa, I doubt you even need a 15W oil, which is required for cold starting temps of 0°C (see your manual for oil weight Vs atmospheric temps chart)
You could use a 20W/50 oil which is only 2.5 (cold/hot ratio ) and hence has less VI's in it
20W oil is good for temps down to +10°C, where as 10W oil is good for temps down to -10°C
Apart from that, Grayracer's post has all the info you need
One thing to remember, Castrol are only bottlers, and do not refine their own oil
They buy there base components on the open market and blend them and bottle them
Is this good / indiifferent or bad?
Also, some oils labelled as fully Synthetic are not.
There was a court case a few years back, (cant remember who). An Oil company was labelling a highly refined oil as synthetic but it was wasn't. A court case was brought against them and they won, based upon the grounds that the public couldn't tell the difference between a true synthetic and this highly refined part synthetic. This set a precedent
I think there are only two truely fully synthetic oils on the market
Motul is one, can't remember the other.
Edited by GuyGraham, July 24, 2014 - 12:49 AM.