Hello Greetings from Serbia... Although i have summer tyres on it i noticed something strange. It seems that right rear wheel is always spining more and faster while on snow, so in driving i have feeling that car is always pulling back side to the oversteer on the right side, i have never noticed this during summer when roads where dry. Can murena differential broke down in such way that it doesnt distribute power as it should be ?
Best regards.
I also suspect that the right rear brake caliper is causing a drag on that side slightly more than the left. Alternatively, the hand brake section on the right is slightly seized and not allowing the caliper lever to return fully, which again will cause a drag that side.
One test is to stop the car and with the hand brake off, go underneath and make sure both caliper levers are fully off against the stops. Then without touching either the foot brake or hand brake, drive again in a straight line on the slippy surface and check for spinning again.
It is extremely doubtful the drag would be caused by the differential.
Although you say the brakes feel free and it is not getting hot that side, it wouldn't take much of a drag to cause a difference.
There is one other possibility since the surface is very slippy with cold and snow and you have summer tyres on the car. The tyre on the right may not be gripping as much as the one on the left. Unless the tyres have a directional tread pattern, you could try swapping the rear wheels from side to side and see how that affects the grip.
Ideally you should fit some winter tyres as these grip much better below 7 degrees.
Roy