Understanding Oils

[Crazy Dazz]

Ok, so my basic understanding of engine oils is this:

SAE J300 defines the grades and viscosities associated with them.
With a multi-grade oil, eg "20W-40" the oil is capable of behaving like the winter weight SAE20 in cold weather, and like SAE40 at operating temperatures.
Generally the higher the number, the greater the viscosity.
So, in theory at cold start you want lower viscosity so that the oil flows quickly??
And at operating temperature you want higher(?) viscosity to provide better protection?

I think I have the above correct, but I'm still puzzled.

Am I correct in thinking that a bigger range is better? eg is 10W-40 better than 10W-30?
OR, if I put in 10W-40, will that be TOO viscous at operating temperature?

And what of the Winter Weights? For example I have an engine that takes 0W-30
"0W" defines Cranking & Pumping Viscosities at -35 & -40 deg C! WTF do I care, since I would be dead?
I assume that number is specified to give certain characteristics at more realistic temperatures, but don't know what they would be.

[Tonz]

Oils aint oils

[fnp]

This video on the subject is worth a look. He explains things very well, better than I could

[oldel]

[QUOTE=Crazy Dazz;6480527

Am I correct in thinking that a bigger range is better? eg is 10W-40 better than 10W-30?
OR, if I put in 10W-40, will that be TOO viscous at operating temperature?
[/QUOTE]
Bigger ranges are achieved with modifiers that break down and you lose protection. They thin out, get diluted and aren't so thick for long.
Nothing wrong with 40 or 50 grades, they just hurt the manufuacturers fuel efficiency tests - so the engines come with thinner oils and that's what is in the manual.
Thick oil does no harm, in vehicles where the maufacturer doesn't care for fuel efficiency it's usually thicker, such as heavy duty diesel engine oils HDEO are 15w-40 and BMW M cars used to have something like 10w-60 .

You don't need anything below the winter grades of 10-15W in Australia, so 0 or 5 is a waste. It's nice to have a thinner oil to protect a starting engine, but if it's a cheap oil then it won't remain thick enough to protect it when it's hot. So a $100 0w-40 would be good, a $30 0w-30 would be bad as the modifiers wouldn't be as good or last as long. For a $30 oil I'd rather use a 15w-40.

You do need to follow the specs of the oil, even if you change the 0-30 for a 10-40 for example you need to use an oil that has met the standards.

[MercuryT]

I think oil type is highly overrated.

Extra oil changes over some special oil spec by far the best option.

I'm not a mechanic but I've been told this many times.

[MercuryT]

Mine is 5w-30 I think.

Mech says could do 10w-40 or similar in high grade and no difference whatsoever.

Just an opinion.