I just registered last night so I might be asking the wrong question in the wrong place... so sorry if that is the case. I have four used Toyotas-- 1999 Corolla, 2000 Corolla, 2000 Camry, 2001 Camry-- Have always used Castrol 20-50 in summer and 5-30 in winter (Wisconsin) since I bought each car. Faithfully change at 3000 miles---- Should I switch to Mobil 1 0W-40 or similar or is that a waste of money?