Nothing makes an engine happier than clean oil for that first spring firing. There tends to be caution with synthetic engine oil, but you couldn't do your engine a greater favor. Run Mobil 1 10W-30 or 10W-40 in your classic Mustang. Lower the viscosity to 5W-20 for your late-model. Motorcraft filters are original equipment in Fords and for good reason: They work.