What oil should I use ford d series?

I have a old ford series d (big old lump) and doing a service on the forklift what oil should I be using with it being an old engine?

also does any one happen to know the oil capacity of this engine?

Many thnaks

So are you changing the oil on a 'D' Series? Or a Forklift? Or a forklift with a 'D' series engine?
I'm confused.
And which 'D' series engine? These ranged from a small 4 cylinder through to two big V8's?
More details?

Both its a fork lift 6 tonner with a series d engine in, and it's 4 cylinder thanks

Cheap old oil with as few additives as possible. Modern detergents in oil can loosen crud inside the engine which can block the oil ways.