Hi Fredmw,
Welcome to the GN Forum. It offers "Newbies" a lot of experience - have a look around through all of the tabs.

While there may be some members who may be able to assist here, have you had a look in the appropriate TLC Forum?
I've been in the Nissan Patrol forum before & have recently joined the Isuzu Oz forum - the wealth of knowledge in these forums is absolutely fantastic.

Perhaps the light comes on early to give you adequate warning that you need fuel soon, bearing in mind that some fuel gauges become inaccurate after 1/4 full. Your 30L left is less than 1/4 full. Most tend to stay reasonably accurate above 1/2 full but drop off quickly after that. I'd like to be warned sooner than later!

Diesel fuel is not only used to burn. Fuel is also used to cool injectors depending on the type of engine before it returns to tank. This leads to a build up of heat if there isnt sufficient fuel. My cruiser on "E" had 30 litres left in tank. My newer D-max seems to have around 12 litres but l've never drained it as it doesn't have a bung.