What engine is it?
My Mondeo is supposed to use a 5w30 Ford Spec oil. During winter I use a 0w30 A5/B5 spec oil for improved flow rate when cold.

I think engine oil additives are a waste of time these days. Things don't sludge up like they used to with mineral oils years ago.

Just pick the right viscosity, use a good quality oil and replace it regularly (I change mine every 6,000miles or 6 months).
A bit of mechanical sympathy won't go amiss too. I try to give my car a minute or two idling when starting from cold to ensure oil is where it needs to be before loading the engine up. I'm pretty gentle on it until the coolant has been at 90° for a good few miles.