Pre-grouping railways had more open wagons than vans and the proportion of vans increased with time. IIRC, the ordinary vans, excluding specials like meat vans, were about a third of the wagon fleet in the 1930s and about half the fleet in the 1960s. I'd guess about 5-10% vans around 1900. Therefore, pre-WW1, almost anything that moved in an unfitted train would be in an open wagon; but anything that could possibly be damaged by weather would be sheeted over. The sheets were also used to restrain loose loads and may have helped avoid pilfering. The oil barrels might even have been sheeted to ward off sparks from the locos.
Inventing loads that don't need sheeting is fairly taxing. I'm planning to run quite a few wagons of building materials, given that my location (London) saw much development in my period (c. 1908). I'll have bricks, roofing slates (Cambrian Railways wagons), roofing beams (on bolsters), stone setts for paving the roads (in the PO wagons of the quarries, possibly from Clee Hill) and probably some building stone (1-plank wagons, either PO or railway owned). Cement may also feature, but that did travel in vans.