Please complete the following information:

• Forge Version: 14.23.5.2847 (MC 1.12.2)
• BuildCraft Version (Add Hash if from github): 7.99.24.7
• Any additonal mods: None (Though, I'm using TCR Classic Refined from Technic, but I did test it with only BC, and the same thing occurs)

To Reproduce
Steps to reproduce the behavior:
0. Be in creative mode

1. Place down a tank
2. Fill it with oil (Using oil buckets, pump, doesn't matter)
3. Take an empty bucket, right click the tank. The tank's oil level will be decreased, but you won't get an oil bucket in the inventory.
4. Go to survival mode
5. Try the same, and you'll get a bucket

Now, in itself it wouldn't be a big problem, you're in creative mode anyways. But, mods like ProjectRed's (Or OpenBlock's for that matter) "Block Placer" won't "suck up" the oil into buckets either.
I've looked at the code of PR's "Block Placer" and it uses a fake player entity, I'd assume that is what causes the issue.

Oil tank + bucket in creative mode doesn't give an oil bucket

That's fully intentional, and not a bug.

mods like ProjectRed's (Or OpenBlock's for that matter) "Block Placer" won't "suck up" the oil into buckets either.

This is a bug, but there's likely nothing we can do about it from buildcraft's side, since it just uses a simple "is the player in creative mode check".

Thanks for the answer.
I wonder what's the reasoning behind doing it like this?
I can't find the relevant code (Nor I have a lot of experience with MC modding), but I wonder if removing that check would solve the issue. Maybe (for whatever reason) the fake players are in creative mode.
I'll try compiling the code myself, maybe that'd (and removing the check) fix it.

Thanks for the answer. I wonder what's the reasoning behind doing it like this? I can't find the relevant code (Nor I have a lot of experience with MC modding), but I wonder if removing that check would solve the issue. Maybe (for whatever reason) the fake players are in creative mode. I'll try compiling the code myself, maybe that'd (and removing the check) fix it.

Mimicking vanilla bucket behavior: in creative mode, filled buckets can place fluids endless times and empty buckets clear fluids endless times.

If you really want to circumvent this; use a tank with a GUI for accepting fluid containers to fill/empty whatever fluid you want. Examples being the Forestry Bottler, IC2 Bottling station, EnderIO tanks, GTCE tanks and so on.