It's not a problem, but the Castrol oil will be twice as viscous at 0°C than the Total. And below that temperature the gap continues to widen, obviously.
So, I'd expect the fuel consumption to be a bit higher after winter starts, but as a plus the heater will function more quickly. I don't think the fuel difference will amount to £70 though.
Do you have accurate fuel data to compare consumption? I believe Fuelly is quite popular in the UK?