More than likely 10-30% but like you mention it most likely is proprietary. If you are going to pay $2qt for std. oil and $2.50qt for blend fine but no more...just buy the full synthetic. For the price the Penzoil Platinum had some good reviews for 5qts @ $20. So a blend must be pretty reasonable in relation to some of the hydrocracked synthetics.
synthetic oil blends
