I'd just add it if you choose to do so. It's a steering box. You could probably add olive oil if you could keep it in there. It's just a worm gear with small amount of use, certainly no heat related friction. The only heat is from the general local to the exhaust manifolds.
Mixing the oil and the grease will just dilute the grease. They are made of the same compound for the most part. A few will argue but non will say their box didn't work.
Cornhead will be closer to true 190w. Bit of oil in the box is fine.