Hi there, the ratings you refer to are the viscosity ratings at either end of the temperature scale. For example, the lower the 'first' number is, the more 'liquid' it will remain at lower temperatures (vital in extreme temperatures with an engine with tight manufacturing tolerances). The higher the 'second' number is, the more resistant the
is to hotter temperatures). You see,
is blended from various grades (what used to be referred to as multi-grades), and due to a bit of chemistry, it was noted that mixing one with the other gave a lower freezing point and an increased boiling point than either had individually. Before then, cars used a 'summer grade - thicker
' and a 'winter grade - which was thinner'.
As far as synthetic vs semi-synthetic vs mineral
, it is generally accepted that for longer service intervals or the use of a
or extreme driving conditions, then a synthetic gives better protection for longer (although, changing your
more regularly than the manufacturer specifies does prolong engine life).
which is rated above & below what your handbook states, will do no harm, however, if you were to use 10w-30w when you should use 10w-40w, then it may cause some problems.
It has to be said though, that in an emergency, I would put any
in as a short-term measure than risk metal to metal contact!
Sorry for the long reply.