Apologies if I appear arrogant - that certainly isn't my intention. Those who know me will no doubt vouch for my mannerisms, and that I mean no wrong. I just get annoyed when some folks post information as de-facto correct, when I know otherwise.
I confess I haven't been to any recent festivals - a life changing illness knocked that on its head. However, until just over 5 years ago, I was a very regular attendee at GTI International, a few other smaller VW festivals, general meets, and JKM dyno days. At that time, the likes of Opie Oils were trying to peddle any 5w40 oil they could sell to Mk5 owners, and whilst 'un-informed' folks were happy to purchase it, those in the know stuck to the official VW recommended oils - even when stage 2 tuned.
There IS a genuine scenario where use of non-approved oils are used - and that would be when an engine is race tuned, and used for competition purposes. The crucial difference that allows non-approved oils is two fold. Firstly, they are used for very limited mileages (and don't have to trundle along in city driving or in sub-zero temperatures). Secondly, the engines have constant access to professional technicians - who are able to strip down engines and clean as required. A racing spec oil categorically has no use in a road car - even if it is 'tuned'.
I am very familiar with Millers Oils - though I don't follow them obsessively - and my illness has put much of my in-depth research on hold. Hence why I asked for a link. I have now found a link -
https://www.opieoils.co.uk//p-115238-millers-oils-nanodrive-cfs-5w-40-nt-full-synthetic-engine-oil.aspx - and this simply confirms my suspicions. Millers Oils are all talk and no trousers - they refuse to submit their oils for OEM standards testing (or they did, but it failed).
IF Millers Oils were as good as they claim to be, they would have armfuls of OEM approvals. If it were as good as it claims to be, it would easily pass the VW502.00 standard - which is the VW Group petrol engine standard for turbo cars on fixed interval servicing. Indeed, many of the old Duckhams oils (even semi-synthetic brews) were approved to VW502.00.
As to different climates on oil usage - that requirement is about 30 years out of date. It pre-dated any usage of synthetic base stocks. Part of the ACEA oil specifications (A3 and/or B3 and higher) require oils to stay 'in grade' - when used continuously at maximum engine revs and maximum load for a continuous 300 hours - at a minimum of 100degC. The HTHS rating requires the same test, but at minimum 150degC. The VW Group LongLife standards are based upon an ACEA A3/B4 rating, with an HTHS requirement, but crucially require max revs and max load for 650 hours. The VW Group LongLife oil standards are a MASSIVE improvement over any of the best non-LL oil standards.
And so the above states why there is absolutely no need to 'adjust' oil viscosity to somehow match a specific climate. The 5w30 viscosity as found in the VW504.00/507.00 oils have been extensively tested from sub-zero temperatures in the arctic, to impossibly hot temperatures in the middle east an Mexico.
Your comment on the Mk4 1.8 turbo is very telling. That engine is a very different beast to the Mk5 TFSI units. The 1.8 20vT is old school - designed to use the VW Group 502.00 oil standard - which was based around a 5w40 viscosity. The oil pumps are different - the TFSI engine oil pump was specifically designed from the outset to use LongLife oils of lower viscosity. I could go on and on over differences in design between the different engine generations, and their very differing oil requirements.
As to VW being 'in bed' with an oil company - well that is a simple fact of necessity throughout the entire motor industry. VW are experts in designing and manufacturing cars, engines, transmissions, etc. VW also specify profiles for the lubricants needed for their products. VW, like all, will offer oil companies to tender for the creation of said lubricant - which is why Castrol has been their official and only partner for engine oil creation. BMW also use only Castrol for engine oil development. So are you really going to state that VW - who will have jointly invested multi-millions of pounds/euros with Castrol - somehow have doubts over the absolute suitability of the engine oils they both recommend AND approve?
Finally, VW Group oil standards, just like all automotive OEMs, freely publish their oil standards specifications (to commercially appropriate organisations, subject to confidentially agreements) - it is a legal requirement under European law! That is why there are around 70 different oils from all over the world that have been tested by VW and approved to meet their VW504.00/507.00 standard. The list of approved oils is regularly updated, and is freely available to professionally registered persons. These lists often find their way onto the internet, so are very easy to double check any 'claims' made on oil bottles/containers. Anyone from a Mercedes background will know of their Bevo portal - which anyone can access for their approved oils.
A black art - but sadly proliferated with snake oil purveyors.
