IIRC there are (very rare) documented US examples of engine failures at extreme low temperatures, due to an unexpected interaction between the viscosity modifiers causing gelling.

In general, oils are (or were) supposed to be mixable, but the ASTMS test requirements for compatibility are very limited.

In terms of manufacturers specification, if both oils meet the spec the mix should too. Mixing viscosities per se should not be an issue. You end up with an intermediate viscosity skewed towards the lower viscosity component, though the exact resultant viscosity is not calculable.

If I had a fancy modern car with manufacturer specifications for oil, which had to operate at low temperatures, I wouldn't chance it.

Since I don't, I mix straight SAE40 1:1 with 15W40 or 20W50. This seems to work very well.

bobistheoilguy.com/forums/threads/engine-after-sit.../

The SAE40 has no VM, so interactions are unlikely, and I never see temperatures below 0C

<< As long as it's above the minimum mark on the dipstick you don't need to 'top it up' yet. There's a widespread misconception that oil must be kept 'topped up' at or near the maximum mark. >>

I don't think there is anything exact about either of the marks on a dipstick. Dipsticks and the tubes some of them occupy will not be made precisely enough for all to represent exactly the same amount of oil. But on any given vehicle they are a fixed mark to show whether oil is being 'used' - provided they are checked under the same conditions: level ground and when engine has cooled.

Personally I am a dipstick-full rather than a dipstick-empty man, but John-F hates the idea of using any unnecessary material, or doing any unnecessary work ... :-)

Checking it, finding it noticably below max, and then NOT topping it up feels like unnecessary work (or at least partly wasted effort) to me.

Also conflicts with my rabid anti-Americanism a bit, since the "widespread misconception" stateside seems to be you shouldnt top it up until it reaches the minimum.

Easy to go below the minimum with that regime.

But of course that wasn't the OP's question.

This was mixing oils, which comes up on the US Oil Obsessive site every now and then, for example

bobistheoilguy.com/forums/threads/mixing-oils.3150.../

Post #11 outlines the downside, though without references. Sieving through other similar threads on there would probably find them, if one could be bothered

Edited by edlithgow on 24/10/2021 at 11:05