That plastic cap wouldn't hurt a thing even if it stayed in there- - - -the gears would have chewed it up if it happened to get picked up with the lube, and they won't even burp. I'd add a couple of quarts of Diesel fuel and run the transmission in neutral for a few minutes, then drain and refill with 90 weight mineral oil like the Ford tractor transmission lube sold by Tractor Supply Co.
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!