I don't have experience with a 540, however, most Lycomings quickly blow out oil filled above the optimal level (optimal meaning what the engine likes or will tolerate, not what Lyc recommends) The fact that yours quickly drops to 7 qts and presumably stays there for a while, would indicate that 7 qts should be your max fill. My 320 recommends 8 qts, but any more than 6 gets blown out, so I fill to less than 6 and add a qt when it drops to 4.
What is your oil consumption rate? Your engine will probably run fine on 2 quarts or less (I speculate that you'll still be pumping oil down to one qt, but don't have data to back that up). If you are burning a quart every hour or two, I would be concerned that 7 qts is not enough for a long trip. This is simple math. If you have 7 qts and your minimum is 2 qts (or use Lycomings recommended minimum level, which is 2.75 qts), you can figure out the risk of running low with your current oil consumption rate.
Assuming proper cooling capacity and reasonable blowby/oil consumption, bigger engines don't need more oil than smaller engines. My 496 CI big block holds around 4+ quarts. About the same as the 180 CI in my wife's Honda.
Larry