Episode: "Dead Freight" (Season 5, Episode 5)
Accuracy: Siphoning liquids only requires a tube, a source, a bucket, and gravity. Siphoning gas from your own car is handy but siphoning gas from someone else's car is illegal (also called stealing). It's so simple that the Wiki How step-by-step guide is 11 steps—and the last step is remove the tubing.
Walt, Mike, and Jesse have just discovered that the DEA has tagged a train load of methlyamine barrels sent over from Madrigal. With the advice from Lydia, the three decide that their best bet is to stop the train and pump the methylamine from the train into buried tanks and replace it with water while the tanks are still aboard the train. There's a lot of set-up (steam shovels, truck drivers, and all that), but the plan goes off without a hitch (sorta...). It's a simple but smart moment of science. Walt knows that the weight of methlyamine is different than the weight of water, and has to work out the conversion rate. This is real science, people.
As for the realities of siphoning, it's mostly a question of equipment. The longer the tubing, or if the tube isn't positioned straight up and down, the more likely it is that air bubbles will form. If that happens, you have to start over.
Is it plausible that Walt and the crew pulled off a siphoning heist? Why not? They had the basic materials: a source, a tank, a hose, gravity, Walt's genius, etc. The surrounding circumstances are unlikely (in real life nobody will offer to help you push your car), but it's not like there's a textbook procedure of methylamine siphoning operations for comparison.