If you look at the specs of 5w30 and 5w40 you will see that the HTHS is the same for both of them so in effect they can offer the same proctection against wear.

The only difference is the ambient operating temperatures, The 30 weight usually has a maximum operating temperature of +45 and even here in the sunny south I can't remember it ever being that hot.

I've used both 5w30, 5w40 and 10w40 in my home fleet and not noticed any diffence.

Interestingly I did notice a difference to how quickly the oil light went out, when using 5w40 of different well known brands. The Valvoline oils took longer for the oil light to go out.

As for mixing or changing to fully synthetic. They are all miscible with one another.

I think alot of the confusion about not being able to use synthetics is due to very old engines oil seals not being up to scratch and them relying on the gunk that has built up over the years. Synthetic oils have very good cleaning properties.

Edited by Wackyracer on 10/07/2014 at 11:31

>>Its perfectly safe to mix all types of oil

Oh no it's not! Try using mineral oil in an engine that's been using vegetable oil (or vice versa). Castrol R's still available and used.

I'd put money on it that a Renault Clio 2 is not using or ever used a Vegetable based oil, Unless some numpty poured it in, in error.

But, Mineral from the last 20 years, Semi synthetic and Synthetic are miscible for sure.