[52 car guy]

So I have this 66 Olds Toronado, the first year of front wheel drive. I am in the process of changing the front differential fluid and am wondering what type of fluid I should put back in it. What came out of it was not gear oil....much thinner.. From what I have read the frist two years 66-67.. this olds used a plantiary system.. then in 68 they switched to a ring and pinion. I looked in my Chiltons and all it says is to use "special fluid number" then it has a number that means nothing to me. Have visited dealerships and no-one seams to know.. thought someone here might have run across this. Any help would helpful

Thanks in advance