Red oxide was another name for red lead (the red colour was an oxide of lead). I think dust and lung disease were involved in the manufacturing process, so it's not possible to buy it any more.
It is possible to buy paint that marketing men sprinkle a bit of red colour in and brand as red oxide. Don't imagine it performs much differently from stuff with a grey pigment.
Try for a zinc phosphate primer for something that sticks to bare metal well (that's what the paints you are looking at probably are if they are any good).
I suspect it may be zinc based so i might look at something a bit more heavy duty as it wont get painted for a while.
Thanks to all
Joe
If you don't plan on overpainting it for a while, use Galvafroid, It's a very thick zinc cold galvanising paint, just google it theres loads of suppliers online, although any engineers or paint merchant should keep it.