On Sun, 07 Jun 2015 16:24:44 +0100, Jim at the Common Riding wrote:

what are the real world problems on switching between the recommended 5w
30 synth blend oil and 10w 40 mineral oil in a car? I want to use up a
load of 10w 40 mineral oil in it. What is likely to go?...Bearing in
mind it will only be used for six months in the summer.

Depends on the engine.

Viscosity-wise, some - especially those that require thin oils like 5w30
- can have all sorts of problems if used with thicker oil.

Base-stock-wise, again depending on the engine, there shouldn't be any
great difference between mineral and semi-synth, except you'll want to
change it more quickly. If it's a turbo, though, I really wouldn't - the
heat of the turbo may very well cause it to carbonise in relatively short
order.

For the price of even five litres of mineral (so cheap) oil, why risk it?
Just chuck it on freecycle or in the lawnmower or even take it down the
tip.