At a guess you would get away with just 3mm difference as there`s more tolerance than that built in the car design but obviously do all the clearance checks when installing.

You are not advised to have two different offsets on the same axle but 3mm isn`t a lot.This may cause the car to under-steer when cornering in one direction and over-steer in the other direction.MOT tester may miss it,police may miss it unless involved in serious accident when their engineers almost certainly wouldn`t miss it back at the compound.

Not as bad as putting two different size tyres on same axle which MOT testers nearly always spot and should fail,I always did.

For eg,a 155 and a 165 on same type rims on same axle is equiv to 5mm offset but with 10mm difference in contact area and will give different cornering characteristics and grip levels between right and left-handed corners. It can also make a vehicle unstable in a straight line.

However if you are taking up stock-car racing you may have made a start to altering your steering geometry to suit,just remember they go anticlockwise!