It is much harder to read spark plugs when using unleaded gas. Leaded allowed for easy color judgments to be made, but a proper burn using unleaded is going to be gray or white. Also, unless you do a proper plug chop (WOT in third, then cut ignition), you are not getting a 'true' reading anyway, even on leaded or race fuel. Basic around town driving is too variable to allow proper readings of the plugs.