Originally Posted by
Big Greenie
Doesn't really make any difference. The 5w number is the one that concerns cold flow properties. The 20 or 30 refer to warm flow properties. They are going to be almost identical in cold situations and the 30 will be slightly thicker than the 20 in hot situations.
That's absolutely correct. Viscocity can be defined as: resistance to flow. The higher the number, the more resistance to flow. Multi-viscosity oils place the cold temp viscosity first, followed by a W for winter(not weight) and then the hot temp viscosity second.
Ambient temperature and resistance to flow are inversely related. This means, the colder the temperature drops, the higher the resistance to flow of oil. Changing from 5W-20 to 5W-30 for an advantage in winter therefore doesnt make sense. A change from 10W-30 to 5W-30 in the winter would make sense because, at cold temperature, the 5W has LESS resistance to flow and can reach vital engine parts faster on cold startup.
If the manufacturer calls for 5W-20 then use it.