I try to turn my crude oil to hot crude oil with the default boiler, but its just not accepting my fluid barrels and everything else.

I think 1.12 has partially fillable canisters so it looks like it should work (in 1.7 barrels refuse to empty into anything that doesn't have enough space). No clue though, you might want to ask on the 1.12 repo about 1.12 problems.