Can use these to get it.

https://mods.factorio.com/mod/dieselTrains
(allows 3 liquid fuel to be used by trains)

https://mods.factorio.com/mod/KS_Power
Gives recipe to make diesel.
Use a (diesel fuel is best, but oils allows) liquid fueled 'Diesel' power generator. (Adds tiny wind turbine, burner, oil burner and diesel generator)

First one is easy, 2nd one, not sure if there are any other 'less content' additive mods that include Diesel fuel recipes.

Hm, are you sure that these mods are compatible with other mods?
In modded factorio there is no light oil and heavy oil anymore.
Adding "Diesel" is ok I think but there are more than 20 burnable fluids in the mod pack, so I would say for my modpack (but sure NigglingBallastLB can see it in another way) if the dieselTrains mod will support all other mod burning fluids then this mod will be ok to add to such a mod pack, the KS_Power is not useful, they are many other possibilities here in the mod pack, but perhaps I get it wrong :)