I'm not convinced that they were running less fuel, that would clearly be cheating and they'd be likely to get found out. I don't think RB would do that.
I might be wrong of course, but I think this might just be comments taken out of context. RB said the issue was that there was no fuel, but they didn't mean the cars were empty, but rather that there was an issue with the fuel pumps that meant they couldn't get the fuel to the engine. At least that's what I think they're saying. But it's odd that it happened so suddenly, so it maybe there is a deeper problem with the car that they're trying to hide. But I'm not convinced they're cheating.