Wa alaykum Assalaam

There is different scenarios here:

a) if oral sex leads to ingestion of madhiy (pre-ejaculation discharge/prostatic fluid), then it is prohibitively disliked and sinful (makrooh tahrimi);

b) if it is probable that one would ingest it, then it would also be haraam;

c) if it is less likely because of precautions taken (e.g. use of condoms etc), then it would be disliked (makrooh tanzihi);

d) if there is chance of ingesting any filth, even accidentally, then it would not be sinful but improper and indecent (khilaaf al-awla).

and Allaah knows the best.