Is Murphy's oil soap safe to use for cleaning mildew off leather car seats?

Yes. Dilute it with water, use a soft piece of terry cloth dampened in the solution to scrub stains gently. Use a water-dampened terry cloth towel to wipe away the soap, then blot it dry with a dry towel. If your owner's manual recommends it, finish with a good quality leather conditioner. You can use this on leather furniture, saddles, boots, purses - any leather with a finished surface. Leather books can be cleaned this way with minimal water, as can leather-topped desks. Murphy's is good for dirty furniture or cabinets.