Baylor U studies have shown that 6-oxo
increases estrogen. I'll never understand why people take this thinking it will prevent gyno. It isn't what I'd call effective for lowering estrogen.
The newest version of MassFX contains aromatase inhibitors, so the likelihood of excessive estrogenic sides are not very common. The ingredient profiles in both the original and Maximum Strength (i.e. newer) versions do not increase aromatase enzyme.... they increase androgens.
To put it simply in an analogy, let's say you are the aromatase enzyme. Androgens are food, and Estrogen is sh!t.

If I give you more food to eat, you will sh!t more. If you start eating Michael Phelp's diet worth of food, you will sh!t a ton. That being said, the body is pretty resistant against sh!t, and can usually clean it up before it leaves a stain on the carpet. You'd have to pretty much fill the room, like increasing your serum levels to a point only obtainable by aromatizable exogeneous anabolic steroids for estrogen to become an issue. Furthermore, if something that increases serum estrogen is preventing gyno, I'd really question if gyno is something that would even occur.