Correct me if I'm wrong but multi-viscosity oils started with dino which had to use a lot of polymer-based viscosity improver to make a thin mineral oil act like a thicker oil at operating temperature.
Along come some of the better synthetic compounds (PAOs & esters) which start off thinner but maintain their film strength and viscosity even as the temps climb.
This goes along with what
Widman is saying, I think.
Therefore, synthetics act a lot like multi-viscosity oils without any viscosity improver at all. Most 'real' synthetic 10W30s habve no viscosity improver, for example. And, when they want to be an extreme spread oil, they use a lot less V.I. than a dino oil would.
Now, does this mean that some real synthetics (not just Red Line's race oils) are essentially straight weights? I don't know enough about the definitions to make this sort of proclamation.
Since Red Line's street oils are essentially their race oils with a different additive package, I would guess that the basic properties they claim for their race oils apply to their street oils as well.
So, why not market their street oils as straight grades? Because it would just confuse the motoring public which use multi-vis oils in all modern road cars. Racers, on the other hand, have historically used straight weights so the labeling (dual-labeling, actually) makes more sense for them.
To sum all my rambling up, I believe the difference between the two types is mostly marketing/labeling.
[ June 09, 2002, 07:56 PM: Message edited by: Bror Jace ]