Never change the style or type of bobbin for your machine. A size 15 and size L are not the same and most can't tell the difference but your machine will. Some machine will have problems if the bobbin has the holes instead of solid rims. Pre wounds come is size 15 or L for domestic machines. Every new machine is tested and set for a certain needles and thread and bobbin. The dealer sells the type of threads, bobbins, and needles used to set the quality control for the brand of machine that is being sold. Use other types if you want to, it's your machine but don't criticize the dealer for only having what is recommenced for best results from the machines.