I have a 62 Jazz.. and have used all three variants. Makes minimal difference - 5/30 uses 0.5mpg ,more than 0/20

I buy purely on price if it meets spec...

In general its a balance between marginal improvements in fuel consumption, and marginal loss of protection, with (probably) a tiny side order of marginal increase in the probability of ring-sticking

Thicker is better for protection (though some people deny that)

Thinner is better for fuel economy

Less viscosity improver (very roughly indicated by a smaller difference between the first and the second numbers) is better for avoiding ring sticking, which can be caused by viscosity improver breakdown.

All the options you list are low viscosity so it becomes a complex balance between tiny effects, so essentially it does not matter .

If you plan to keep the car a long time (or suffer from an excess of mechanical sympathy) use the 5-30, If not use the 0W-20, reap the tiny fuel economy benefit, and feel all cutting edge and trendy.

Or just use whatever is cheapest and most readily available.

Edited by edlithgow on 31/08/2020 at 02:21

I would say keep using what's been used up to now, assuming you know what that is, and assuming it meets the spec.

Say you get 50 mpg and do 10000 miles a year.

That''s 200 gallons. 1 % better fuel economy (probably about right) is 2 gallons, which I think is currently about a tenner in The Yook?

I'd guess 0W20 is more expensive than 5W30, since it'll need more genuinely synthetic base stock to achieve the 0.

If its a tenner more for a sump full, you break even, less you gain, more you lose.(assuming annual oil change)

So there's probably not much in it,

Or my mental arithmetic is off, a real and present danger.

If there's a grade stated on the oil filler cap, then use that. Unless someone here has a definitive answer (noting that different engine spec may use different spec oil), then ask at your local main dealer.

It appears to be more commonplace for newer designs of cars to use less viscous grades of oil, presumably so they can achieve higher mpg because of lower frictional resistances on the engine's moving parts.

That doesn't mean we should change to such a oil grade. To be honest, most car handbooks I've seen will give specifically what one (or ones) you can use for each specific engine spec covered in the handbook - and some include oils used in a specific engine when under different external conditions in a range of countries.

Be careful not to read one of these (e.g. for hot and dusty climates such as Australia or the Middle East or colder ones in Northern Europe) as what is acceptable in the UK. The same goes for service intervals and what gets looked at/changed during services.