i think that is the idea - the centre viscous diff can get overworked
e.g.
[
www.subaru-impreza.org]
or offical line from subaru.com
"A small difference in wear isn't going to do much harm, but subaru.com has this to say:
On All-Wheel Drive (AWD) vehicles, it is extremely important that the rolling or outer circumferences of the tires be within 1/4 inch of each other. This means that you must physically measure the size of the tire. This is best done with the weight of the vehicle off the tire and at the tread centerline. Also be certain that the tires are properly inflated since this can affect your readings.
If you need to replace just one tire, the same holds true. The measured difference in circumference between the replacement tire and the other tires on the vehicle cannot exceed 1/4 inch.
Depending on the vehicle mileage, it might be better to replace all four tires.
If the vehicle mileage is low and the tires have been rotated and driven at the proper inflation pressures, then you may be within the allowable 1/4-inch difference in circumference.
If the difference between the new/replacement tire and the current tires is within this 1/4-nch maximum, then the tire should be compatible. If it is not, then other tires will need to be replaced to conform to the 1/4-inch rule.
So, in terms of tread depth, 1/4 inch in circumference is about 1 mm of tread. The difference between fairly-worn and unworn can be a lot more than that... "