So oil sump light came on, checked dip stick and confirmed (engine cold), low level, first notch.
So went and got 5l of oil and put that in, second notch. Light has stayed on.
Went for a drive and when back, left for 30mins to check dip stick, now far to much oil.

Does my truck think it's April 1st or what gives?

Difference between low and high is about a litre or so, so 5 litres is way over

Don't drive it until it's drained

It takes a fair while for the oil to drain down from the head to the sump. Definitely drain the sump down before you drive as Greigboy says or you can do some pretty nasty things

Daz, under the oil filter there is the oil switch, they often leak through and cause the light to come on, you'll only see it from underneath .... also check the spade terminal is fitted properly / connected, and some have the female spade screwed on to the sensor, check it's tight..

Thanks Lez, I'll have a gander we I get free.
Drained the oil and realised that
5l of fresh oil with old oil = draining off just over 4litres of useless oil = bye bye £20 odd