Atlantic Aviation Motor Oil

Please help! I'm new to sign collecting and made what I think was an educated buy which hit my interests in aviation and petroliana. I found it in a Pittsburgh antique store. The store owner got it just before I walked in and he decided to sell it only because he bought 2 and he could keep the other. He said he hasn't seen this sign in the 35 years he has been collecting and his asking price reflected that! He also said he was going to ask 50% more for it if I didn't buy it... Was that a slick sales technique??? The condition is very good but dirty. I picked the cleaner one; it may not be the better one but it was the one I could assess the best. The sign is approximately 48" x 18" made of tin with a wood frame. The "TIO" in the word "AVIATION" is a bit scratched but definitely very readable. Everything else is just covered with grease and dirt and I can't wait to uncover it! The wood frame is in great shape with metal support on the external corners and it has the original eye-hook mounting hardware on the top. I searched the internet and I'm having a tough time putting a date and value on it. One just like it sold at an auction in 2005 and I could pay $15 to see what it sold for but I don't think it's recent enough to be accurate. My guess on the age is early to mid 20's and I hope it's worth what I paid for it! Does anybody know?