I pulled the air charge sensor and it was black with thick crud/oil deposits. Greasy looking. Is this normal for a 13 year old factory installed sensor?

('91 F150, 4.9L, 161,000)

 

yes it is. Clean it with carb. cleaner and put it back in. Your whole intake system is likely full of that black crud caused by oil vapours condensing there and no fuel going by to clean it off (as in the old carb. or throttle body injection days). The main problems are the throttle body getting gummed up, the IAC getting gummed up, and your sensor getting gummed up. Other than that the only other problem is slightly reduced air flow in a badly gummed up system.