I am thinking about changing over to synthetic motor oil. I tow a 5th wheel camping trailer about 225 miles round trip 2 to 4 times a month from April to Oct. and I change the oil every 3 months or 3,000 miles. If I switch to synthetic will I still have to change it every 3 months or 3,000 miles or can I go longer? The trailer wieght is 8,300 LBS. dry .