hello, 1992 acura legend 3.2L V6- 75000Km's. vehicle is parked outside and endures a 4km trip to her office every day. occasional use on weekends. I use the cheapest 5-30 non-synthetic oil I can buy and change it (with a honda filter) every 4000km's. should I go to a synthetic or am I ok with this setup? also, just purchased new toyota echo. will see about 25000Km's/yr regular use by me. mixed city/hwy. is it safe to stick with toyotas OCI for regular driving with non-synthetic or should I use severe service schedule. I change my own oil and enjoy it so am not disinclined from doing it more often. just don't want to be needlessly wasteful. opinions, recommendations welcomed. thanks.