I have used evapo rust on a gas tank that had been sitting 54 years. I let it sit for 3-4 days maybe longer and it did a decent clean out but did not get all of it. So I buy 3 gallons of white vinegar and out that in the tank and let it sit for 3-4 days and it took out the rest of the rust. I bought a '73 RT360 that had been sitting for 7-8 years and the tank was rusty on it as well. Filled it up with vinegar and went to work for 14 days, (14 on 14 off) and the tank look factory new inside.

So I would not be afaid to use vinegar on the oil tank and rubber seals at all. Dont get in a hurry, 3-4 days check it, dump it out rinse it out and if there is any rust still in it fill it again and give it time to clean it out. 3 gallons of vinegar is maybe 6-7 $. Thats all I will use now. Fill it full and put the cap on it. Good luck