On 27/07/2020 07:38, Thomas Prufer wrote:
On Sun, 26 Jul 2020 13:48:44 +0100, Another John wrote:

Cheapest was a synthetic oil. I asked the lad there (an about-40 yrs
old lad, spent all his working life in the motor factors, and engines)
about mixing this with the oil I'd got: he said "doesn't matter - just
shove it in, as long as it's the right viscosity".

ISTR: there was once a time where the different additives in engine oils could
be incompatible, and cause it to gel or thicken. Along came the military, and
mandated that all oils be mixable without (drastic) downsides.

(I tried to find my source, but google groups seem more broken than usual,
returning 0 hits for "motor oil mix"...)


Thomas Prufer


Some time ago now. I'm not sure whether it was driven by the military,
or the motor industry, or just the oil industry getting its act together
(given the reputational loss if one was rumoured to be making an
incompatible oil).

My understanding is that all motor oils are either mineral, or semi
synth, or fully synthetic. Synthetics are all group IV,
polyalphaolefins, i.e hydrocarbons and therefore safely miscible with
mineral oils. Having all the base oils compatible means it makes sense
having compatible additives across the brands.

If you need higher performance, e.g. military jets, high performance
compressors, you use Group V base oils which include phosphate esters
and poly alkylene glycols. You *do* have to avoid mixing these with
other commercial mineral oils. I'm not sure how much this is down to
incompatibilities of the base oils or incompatibilities of the additives.