Not too old. You are perfectly fine using a full synthetic oil except, as mentioned, for breaking in a rebuilt engine. High ZDDP synthetics are available for older and high performance engines.
What do you think about the Z-ALT additive? Snake oil or is it good for my motor?
As Bill mentioned...snake oil. Modern oils, especially synthetics, are precisely engineered with an additive package to handle specific applications. Supplementing with off the shelf additives adds unknown chemicals and can actually cause problems...particularly the ones that contain chlorine.
