Perfect Parkin

Hello, I'm making the Yorkshire parkin recipe from Delia's trusted old Complete Cookery Course, which I've been using since the 1970s, but never made this recipe before. She gives the option of using a mix of golden syrup and treacle, but states that dark golden syrup is better. What is that? As I'm in the USA, and the standard Lyles is very expensive here, I make my own. Would the dark version be the same recipe with dark brown sugar? I don't want to try it and waste loads of sugar if this is wrong!
many thanks,
Sean