new car

So I have a new car (new to me -- it's older than my son), a Volvo station wagon with a turbo engine. The mechanic says it needs at least 87 octane gas, but the woman I bought it from always used 85 octane and never had problems. The mechanic says "Maybe, if you never put your foot down." I never put my foot down. Should I use 87? I live in Colorado.