...I have read about different viscosities being better for different times of year I.e. summer vs. winter temps (I'm in Scotland)....
Cheers!
The two numbers in an oil spec are for viscosity values in winter (the W bit) and in summer. So a 5W-30 has a winter viscosity index of 5 (it's 'runnier' in the cold) and 30 in the heat of summer. All (so far as I know) car engine oil has been 'multigrade' in this way for some decades now. And most of Europe, from the cold north to the warm south is covered in one multigrade band. These 'bands' have got narrower over the years as the tolerances in engine manufacture have improved. Thinner oils made from synthetic products rather than mineral oil lead offer greater protection using a less 'thick' lubrucant and so reduce fuel consumption and emissions too.
The crucial part (for any modern car these days) is the second part, the ACEA 'C' rating. Different engines, especially since the advent of catalytic convertors and particulate traps (which some petrol cars now have, as well as all modern diesels) require different C ratings to suit their particular set up, and using the right viscosity grade but wrong C rating can cause issues (more especially with diesel particulate filters, it has to be said)
Fiat's TwinAir engine seems to be the most fussy design in recent years when it comes to engine oil. As well as needing the grade
and C class, it seems to really only be really happy when using the very specific
brand of oil it was designed for and calibrated with -- that used to be made by Selenia Oils (a company owned by Fiat many moons ago) but now part of the Malaysian Petronas group. Technically car makers are not allowed to specify a
brand of oil to be used -- just the ratings needed, and a 'we
prefer' or 'we
recommend brand X' comment (the same laws mean they cannot insist on dealer-only servicing either...)... but the TwinAir (which uses oil pressure to lift the valves in the engine) seems to be easily harmed by the wrong oil choices.
Pre 2015 (Euro 5) models were quoted as needing 5W-40 ACEA C3 oil, and Fiat recommended Selenia K Pure Energy.
To confuse things further, Fiat changed the required oil viscosity rating somewhere after 2015 to a thinner still 0W-30 for the TwinAir (when the Euro 6 version of the engine came out). And a different C rating too: 0W-30 ACEA C2. The recommended oil changed to Selenia Digitek Pure Energy.
What's not completely clear is if that applied retrospectively to the earlier cars or not... They also halved the recommended oil change interval, from 18,000 to just 9,000 miles (or annually if that came up first). This in part was owing to a number of engine issues caused by low oil levels, with some owners thinking they could get away without opening the bonnet or visiting a garage for a long time, up to 18,000 miles.
My first few cars (starting in 1982) all 'just' used 20W-50 oil – any brand, and with no complicated extra codes after the numbers...
Nothing seems to be simple these days...
