Hypothetical mythical motor oil that doesn't change in viscosity with temperature, at least from -20C to 100C?

Semi ask science? This might sound like shitpost, but I'm genuinely curious.

If such motor oil existed in affordable quantities, would this be cool, or actually worse?

One thing I can think of is engine parts and thermal expansion. Like maybe when the engine is cold, there's slightly more wriggle room between parts and the thickness of cold oil would suit the space better. So for a machine expected to have frequent cold and hot cycles, and if designed with absurdly tight tolerance, "static viscosity oil" would be paired with "materials that don't expand or contract", further driving the cost, when we may as well used "normal oil and materials".