That is my view too!
Barrie
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My view also unsubstantiated is that there is no correct spec.
Mazda recommend the use of a certain spec in a certain market with a certain service interval. The individual owner’s care or neglect along with known issues like regular short journeys and thrashing the engine before the oil is warmed up will also have a major effect. I use a fully synth 5w-40 just to be contrary and nothing has yet happened.
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Agreed that regular changes of cheap oil is better than stretching the miles with an expensive oil.
But did some digging and the online MX5 NC manual states you must use SM marked oil. This means API SM. But here in Europe we don’t always display the API (American Petroleum Institute) numbers. Does anyone have access to the spec Mazda UK (or Europe) state for their oil requirements, does it still say SM?
Some oils over here do display that, and the on line ‘find correct oil for your car’ does return oils which are API rated SL which is a concern.
The SM standard is newer than SL, and appears to have testing process’s that take into consideration maximum Cam and Bearing wear rates. The latest is SN (or SN+) which is backwards compatible.
API charts, like this linked one below show that SL was superceded for engines after 2004. So I want SM in my 2008 NC.
http://www.pqiamerica.com/apiserviceclass.htm
Castrol Edge Titanium FST A5/B5 is only SL rated. Where as the same oil in C3 flavour is SN rated.
The cheaper Castrol Magnatec C2 is also SN rated. So cost and marketing effort do not equal best oil for your money.
So, I reckon going forward I will only be using this stuff. It is as cheap as chips, UK made, fully synthetic, ticks the SM box for my MX5, the C3 box for my CX5 and the LL04 box for my BMW140.
So has anyone got any horror stories about Kerax?
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Interesting.
“Personally” as long as the oil meets the spec and the grade, does the cheapness come into it…
No matter what the cost, if it does not meet the criteria/specification I wouldn’t use it.
KERAX.
Personally, never heard of the Kerax brand.
No idea what it costs,(how much average cost by the way) but if you like it go for it job done` 
EDIT- Now I see why! yes very cost effective.
(The oil I looked at was SL, your picture is SN)
You got me hooked into this one and researched the company etc.
Now British based in Birmingham.
As you say are there oils cheap or priced about right and are the other well known brands over priced.
Think I will give it a go to be honest.
Thanks for the information. 
I wouldn’t get too hooked on the API oil specs. I used to take note of them, but a quick look at the ‘Lubrizol Relative peformance tool’ shows that they (even the latest SL/SN) are actually very basic specifications. It was commented on here by Opie oils recently that almost any oil would meet those API specifications. The performance standards of ACEA oils A3/A5 etc, even from a few years ago, appear to be significantly superior in most aspects. Most manufacturer specifications also would appear to be well ahead of API, although Mazda appear not to have gone down that route.
JS
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Only the best for mine… and frequent changes.
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“…almost any oil would meet those API specifications.”
Then why do many not meet SN spec? Look at markets where API is important and you will see lots of ‘top spec oil’ are SL. I am no expert, only keen to understand what is best for my car.
“Only the best for mine”
I would agree, but define “best”. Most expensive? Most consistent across batches? One you can fry chips in and they taste soooo good?
This is what I am trying to get at, what is best for my car. To me that is in the manufactures specification and that appears to be 5w20 or 5w30 API SM.
I am aware there are specific Ford oils but these appear to relate mainly to diesel engines, and I can’t find any specific requirements for the engine in any of its disguises (Mazda, Ford or Volvo).
I am probably over thinking this, it is what I do, but I don’t believe in paying through the nose for something when there are cheaper AND better alternatives.
Well, you have asked the questions that I was going to ask, but thought it best coming from you.
I am with your thinking all the way, unless we have a real true expert on the forum that can give the pros and cons by way of evidence and not opinion of all the different brands.
Now that is a tall order…
Personally that is WHAT I go by and the cost/best/brand is largely irrelevant in my opinion. 
People will say one brand is better than another… But is it and why?
They (I certainly don’t) wouldn’t know unless they made it, as additives in Fully Synthetic oils (as other grades) are pretty much a top guarded secret I suspect! 
Is Castrol better than Mobil, Total, or whatever?
I doubt it, as the boffins/chemical labs work hard to get it right and test it to death over a good trial period before it ever comes on the market.
They just could not afford to get it wrong for obvious reasons.
Companies will all give the sales blog which is what people cotton onto of course.
The vast majority of manufacturers will provide a lube advisor for each car anyway.
It really is not worth worrying about. Change oil regularly depending on yearly mileage and you will be dead before there is an issue. Doesn’t matter a flying f♧◇♡ what you use. The issues come from neglect and low oil level. Anything else is just noise.
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That’s the end of that topic discussion then 
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Which reminded me to get me digit extracted and book the Mk1 in for it’s annual ( or 4k miles-ish) oil & filter swap.
I’d not want him calling Esther’s helpline to report neglect. 
That was priceless really! 
Excuse my rant,it was 6 pints of lager induced.
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The grammar was still top notch though
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Love to be with you in a pub after 8 or 9 then! 
Eight or nine pints those were the days just a distant memory now heck I’d be in the loo all night if I could still stand up that is. 
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Love to have few beers with you lot.
Maybe some day. 
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Greater range of temperatures across the USA even in individual States so oil spec adjusted accordingly.
You would usually expect a climate with hotter and cooler temperatures to have a wider range (0w40 perhaps) than a narrower one 5w20.
I suspect the spec of a lower viscosity oil has more to do with mpg and power figures than climate in this particular instance