I've tried thinner oils in my 1981 302 w/ 98k miles but they all foul the spark plugs. I suspect "leakage" caused by worn rings or rings w/ clearances designed for thicker oil. Ford called for 10W-40 back in 1981. THe use of xW-30 and xW-30 synthetics would foul the plugs in 30 days or more. I've replaced the valve stem seals already but the use of xW-30 would eventually cause a no start condition. A few ppl suggested I tried synthetic 15W-50 in the motor year round. Any ideas on this?