I would be hesitant to think 90 weight would combine well (or thoroughly to be exact) with the thicker grease (whatever that grease may be). Joe might have thoughts about this and we should see what they are. In an ideal world I would remove the box, clean it well and inspect things while I did that and replace the pitman shaft seal and gaskets, use thread sealer on everything else and re-adjust it. In a moment I'll mention something else about that.
As for the grease being brown...If somebody used the old style of wheel bearing grease in there or even the old style of chassis grease, it was naturally brown, got darker and thicker with age and it may have been sort of reddish in color. I have a couple of cans of the wheel bearing grease left here by the way. One is 50+ years old. We're having trouble with images it seems but later I could post some images of that grease so you could see it. It very well might mimic grease mixed with rust...especially one of the cans. This is what is called long fiber grease and not at all good for a steering gear. Some of that stuff actually looks and feels like somebody mixed hair in with it.
I don't mean to come down too hard on using corn head grease. A lot of people here seem to have convinced themselves it is the answer. Herd mentality isn't always correct as we all have learned but it can be compelling.
I was thinking while having a cup of tea and I don't have any idea how many steering boxes I've seen. Over 20 easily. A few of them simply could not be used because somebody had used the wrong grease or too little lubricant in them and some had harbored water (very bad). And we can't un-scramble that egg. No way to know what somebody did long ago. The wear on the sector gear of one was literally the stuff of highway nightmares. I'm not sure how anyone drove it in anything resembling a straight line. No adjustment was left at all so the wheels would just pick their own path.
If you do take out the steering box and make a project out of re-doing it, please automatically replace the pitman bushing and check the shims on the adjustment side. Because the pitman shaft/sector gear is predominately limited to partial motion under stress, that bushing wears out in an egg-shape and that in my experience makes it harder to seal that area. You know, the lubricant runs toward the bottom/top as you'd expect it would and the more it wears the more the shaft bounces up/down and leaks. The balls don't always require replacement and the top and bottom worm bearings are often pretty much okay with adjustment but I'd look at them to be sure. I hate to say it but those bearings in worn condition might be better than the offshore made ones you find today. Once you remove the sector gear just turning the worm shaft a bit can reveal a lot.
