50 Cent Trolls Diddy With 'Letter' to Judge Ahead of Sentencing

50 Cent's screwing with Diddy a day before the mogul finds out whether he's headed off to prison or if he gets to go home ... telling the judge to think of the general public's need for baby oil.

The rapper shared the message on Instagram Thursday afternoon ... kicking it off by addressing Judge Arun Subramanian directly and mentioning his longstanding beef with Puffy.

50 claims he's feared for his life multiple times over the years, and he wants the judge to think about the "safety of the general public" when it comes to his impending sentence.

Diddy hasn't had enough time in prison to learn the error of his ways and truly reform, 50 explains ... regardless of teaching "Free Game with Diddy" -- a business class -- behind bars.

Diddy's son Christian Combs also catches a jab from 50 in the message ... after 50 Cent mentions the rape allegations leveled against him in a civil lawsuit.

While it seems to start out somewhat genuinely, 50 Cent makes it clear he's trying to get under Diddy's well-oiled skin with the message ... 'cause he also asks Subramanian to think of the world's need for baby oil.

He writes, "Diddy's only going to hire more male sex workers and keeping most of the baby oil from the general public. And babies need it! My Netflix doc on this scandalous subject is coming soon!" He also shared a photo of the judge.

We know federal agents found a veritable ocean of baby oil while raiding his homes ... though we'd venture to guess the world's supply could survive if the judge gives Diddy time served.

Remember, prosecutors have recommended the judge hit Diddy with more than 11 years in prison ... while Diddy's asking for something closer to 14 months, with credit for the 12-plus months he's already served.

50 trolled Diddy throughout his federal trial ... sharing photos of Bert & Ernie when "Jane" revealed those were their nicknames for one another.