These are the corresponding test temperature range for the number before the W
0 = tested to remain fluid * -30 to -40 degree C
5 = tested to remain fluid * -25 to -35 degree C
10 = tested to remain fluid * -20 to -30 degree C
15 = tested to remain fluid * -15 to -25 degree C
20 = tested to remain fluid * -10 to -20 degree C
25 = tested to remain fluid * -5 to -15 degree C
We never experience those Winter temperatures in the Phils so we should not care about the numbers prior to W
-20 to -40 degree C is already extreme (in my field of work, it will trigger an automatic shut down of the machine)
Also at -40 there is no reason for the engine to start or continue running unless it is being used as genset or snow mobile in the polar region... heheh.

For the multi SAE grade XXW-YY, the XX is the base added with viscosity index improver (additives).
The base is the lubricant while the additives are added for the main purpose of improving the viscosity index and for additional function/selling points like cleaning capabilities and etc.
The lesser the number prior to the W the thinner is the base lubricant being used in the oil mixture. The corresponding test temperature range will give us an idea how thin is the base lubricant in order to remain fluid at extreme temperatures.

Oil is used in the engine for the main purpose of lubrication, cleaning should be secondary and being able to perform during winter is none of our concern or not applicable in the Phils. heheh.

What we should consider is the availability. Since manufacturers are also aiming for a universal market or a "one size fits" all product. It is expected that the most abundant oil or the oil grade in competition is in the mid-range 10W-30, 15W-30, 15W-40.

Also there are regulatory agency like API that sets the standard for the quality. So instead of going deeper to the mechanical and chemical properties, what we should be looking for is the API rating like:

CI-4 for high performance diesel engines with EGR (having EGR problem? might be due to your engine oil)
or
the current highest rating of CJ-4 which is recommended (or specially developed) to help protect both the "clean diesel engine" and to protect the diesel exhaust after treatment system like the diesel particulate filters (DPFs).

IMO SAE 15W-40 with API rating of CI-4 or CJ-4 is ideal for diesel engines in the Phils.
Of course subject for availability, price, reputation of the manufacturer and the dealer.

See the link for the list of oil manufacturer and brand names (as recommended by a car manufacturer)
click this link

Dapat pinagkakaabalahan naman paano malalaman ang genuine sa fake.