I recently purchased a used Bunton walk behind to use to bag in the winter. It ran fine for a few weeks, but now runs very slow, and doesn't seem to want to go up hills. My first thought was a loose belt, but when I replaced it, same thing. I have checked the fluid for contamination, and it looked clean. The oil in it is(as per the manual) 20W50. Could that be too thick for colder weather? If I do switch to a synthetis Mobile 1, do I have to purge the system, or just empty the tank and fill it with the new oil? Any advice would be much appreciated.