It could be a combination of heat from engine and fuel pressure too high for the plastic filters. Plastic filters are generally suited for gravity fuel feed or very low pressure. I did not measure the pressure on mine but I had initially installed a plastic inline filter and it immediately began to leak. That is why I chose to go with the metal filter.
I used Royal Purple 10w 30 exclusively in my Coleman 550 for the entire time I had it. Changed the oil twice a year. After changing out the fuel lines and adding the 10 micron fuel filter, it was problem free for me. I put 1,300 miles on it exclusively on my property here in east texas.