<< I do not follow why vauxhall "longlife" says 1 year regardless of miles and the container says 5 year shelf life. >>

It's a bit of a misnomer. It really means 'long distance'. Oil, like most other commodities in tins or bottles, does not go off after 365 days. The 'one year' advice is purely for the benefit of the garage trade.

Shelf life means what it says - sitting unopened on the shelf at ambient temperature. Under those conditions nothing happens to oil in decades. After weeks and months churned round in a hot engine it's chemically different.

For a rep's 20000 miles a year on mainly fast roads at an overall average of, say, 40 mph, that's 500 hours of churning - three weeks. That will indeed affect the lubricity. But if a pensioner averages 30mph over a mere 5000 miles, that's only 167 hours, less than a week. Assuming the engine gets fully warmed (modern small engines get warm very quickly), there is clearly no need to change the oil yet. Tractor engines, which probably work harder than car engines, do around 500 hours between changes.

Whether on a shelf or in a virtually airtight sump, oil won't 'go off' like it used to do in old cars when it was contaminated by moisture and micro-organisms. Older drivers will remember the characteristic rotting oil pong when following one of these!

Edited by John F on 07/06/2020 at 10:06

i .quote your info

These examples show viscosities at different temperatures:

Grade................0degC............10degC...... .........40degC...............100degC

0w-40...............665cst.............354cst........ ..........82cst................14cst
5w-40...............842cst............ 430cst..................91cst................14cst
10w-40.............874cst.............440cst .................91cst................14cst
15w-40...........1260cst.............595cst........... ......107cst................14cst

And your point is what ? All the grades match at 100°C, as indicated.