"Are composite oil cans worth collecting and if so is it the graphics that drive them as it is with the metal cans? What is the “normal” value of these can?"
I think they are worth collecting......I've always said if it's a round quart can, it's something that hasn't been widely produced for a few decades now, so it has some value as a collectible item representative of a different era. When I started driving in 1992, I could only get oil in plastic containers around here, no popping the top of a can although I do still remember round quarts when I was really small in the early 80's. Obviously the majority of composites aren't that valuable, but there are some graphic ones that have higher value. I'm not one to attach values to anything on Old Gas as it's pretty subjective, but IMO the plentiful/general composites are $5 - $10 when I see them. It's definitely a good area to start collecting on a budget. I have a Purple Martin composite quart that is graphically one of my favorite cans.
"Are 2 gallon cans becoming collectible?"
I think they've always been collectible, but in the last few years the prices on many have certainly increased. "Big Cans", like 5 quarts, 2 gallons, 5 gallons and drums were once something most collectors only wanted to collect "their brand" of. For me, that would be ANY size Mobil can with Pegasus on it. Most people say they take up too much room to collect larger size cans specifically, meaning collecting every 2 gallon can you see instead of just what was produced for your specific brand(s). But it sounds like more folks are collecting just 2 gallon cans and even 5 gallon cans now. I've always felt that the larger the can, the harder they are to find in nice condition. Many were repurposed around the farm/home and were beat to death. Maybe the larger cans are finally getting the attention that they deserve, especially for nice examples. I've always liked the larger cans myself.....they make for an impressive display when they're clean and displayed in a group.