Just thought I would add to the discussion the specific product you are looking for won't exist exactly because in this industry the term high mileage is associated with seal conditioning usually and you don't want seal conditioners.
Just so you know all properly formulated motor oil contains seal conditioners and high mileage oils just contain a larger dose, I've heard it's roughly double dose for high mileage oils verses their conventional counterparts.
If you want a different additive like a performance additive they exist, but liqui moly motor oil saver is probably the closest thing to what you asked it's a high viscosity product with seal conditioners.
If you want a low viscosity product with seal conditioners try the ATP 205, it's very thin like a mineral spirit from what I recall, the ATP 205 helped my high mileage Nissan greatly with leaking seals in a very short time.