As well as the linked article, there are lots of other 'tech articles' from Opie worth a read (I have just read a few, as I had nowt else to do whilst I had a cuppa!)....
.... as with many such things, most of it went over my head, but a few things make sense.
The discussions on viscosities took me back to 'A' level physics practical where I was given a gallon of oil, and had to drip it through a pipe at various temperatures and plot the results on a graph - but I'm buggered if I can remember what the outcome was, or what it all meant.
I started out thinking that I *may* be causing problems by using 10w60 (Fuchs Titan oil) in my car.... and that the article (and many others) talk about modern engines needing thinner oils.
The more I read, the more confused I became about what is a 'modern' engine - they talk about 'new' engines, designed for thin synthetic oil, and they talk about 'engines from the '70's which required 20w50 mineral oil.
Clearly, "our" engines were designed, built and specified in the 1980's, and to my mind that puts them between the 'old' and the 'new', as they came out when synthetic oil was gaining popularity, and Lotus designated an oil (grade) which suited the engines when new.
The biggest surprise to me was seeing the huge difference when 'cold', vs 'hot'.
From everything I have read, I believe that 0w40 is a great grade for a new (condition) engine, and is probably best for a lower mileage unit, for everyday road use.
BUT
I am currently using 'thicker' oil as my engine has over 140K miles on it, and it appears to keep a good pressure for me and my use.
I tried to find some scientific proof to suggest that I am right to use it, and came up with the fact that I get lower airflow over the sump surface to aid cooling due to having a sump guard, so it will run hotter, and 'need' the thicker oil
(ignoring the fact that we have an oil cooler radiator).
I have previously used 0w40, 4w40 & 5w30 grades, and didn't feel like I was getting good oil pressure, and may be causing damage (which seems to be correct on a hot engine, but not so great on a cold one)... so I remain confused.
The one thing I am sticking to is that everyone has their own reasons for their choice (even if it's just price/availability, and not science), and my choice may not suit you at all.
I may well try dropping the "W" number next time (but 5w50 doesn't give the same 'pressure' at hot temperatures as 10w60.... but is 'better' at cold) as I've not used it before, but probably wouldn't go to a 0w40 as it didn't seem so good in my engine last time I used it.
Basically, if you stick to a good oil, somewhere close to the specs, it is a good starting point, and then try a grade in either direction and see how it feels for you and your engine, with the use it gets.
Opie (and other good suppliers) will happily give you a base recommendation for your own circumstances.
(Certainly, with the T5 there are more opinions on the forums than members, but I always use Quantum 507 and it hasn't blown up yet, likewise in the Vauxhall I stick to GM oil, and in the Triumph, it's good old Castrol GTX 20w50)....
by Elantastic » Wed 05.05.2021, 17:19 
