I have used a heat gun & PB Blaster, Liquid Wrench, etc. after a little warm-up heat and then let it set, sometimes overnight. It usually will break free when I threaten it with my big honking crescent wrench!
You should clean both float mechanisms (fuel gauge float & the aux refill switch mechanisms) while you're at it. Yes, the Aux fuel sender switch can be a problem if it sticks and activates a refill of your day tank from Aux tank. It will cause an overflow & can make a big diesel mess should it stick at the fill trigger level & overflow day tank. Ask me how I know... lol
I usually don't enable the automatic Aux refill until day tank gets low & I turn it off when it's topped up. On one of my gensets I have a 50 gal Aux tank and at today's diesel fuel prices that's potentially a very costly fuel loss & BIG mess. Especially if it happened when no one is around or left with aux setting activated & running overnight.