How do I read the numbers around the 'W'? For example 5W40?
As oils heat up, they generally get thinner. Single grade oils get too thin when hot for most modern engines which is where multigrade oil comes in. The idea is simple - use science and physics to prevent the base oil from getting too thin when it gets hot. The number before the 'W' is the 'cold' viscosity rating of the oil, and the number after the 'W' is the 'hot' viscosity rating. So a 5W40 oil is one that behaves like a 5-rated single grade oil when cold, but doesn't thin any more than a 40-rated single grade oil when hot. The lower the 'winter' number (hence the 'W'), the easier the engine will turn over when starting in cold climates. There's more detail on this later in the page under both viscosity, and SAE ratings.
A quick guide to the different grades of oil.
Fully Synthetic Characteristics
0W-30
0W-40
5W-40 Fuel economy savings
Enhances engine performance and power
Ensures engine is protected from wear and deposit build-up
Ensures good cold starting and quick circulation in freezing temperatures
Gets to moving parts of the engine quickly
Semi-synthetic Characteristics
5W-30
10W-40
15W-40 Better protection
Good protection within the first 10 minutes after starting out
Roughly three times better at reducing engine wear
Increased oil change intervals - don't need to change it quite so often
Mineral Characteristics
10W-40
15W-40 Basic protection for a variety of engines
Oil needs to be changed more often
Read more:
http://www.carbibles.com/engineoil_b...#ixzz33hn8uL5f