Working in Frankfurt next week and thought I may check out the FKK's. The Palace and The Oase have been discussed here. Any advice or suggestions for a first timer?
Have been to both several times. Either one is great. Palace has a more urban feel, like a night club. Oase has a more rural feel, and is particularly great in the summer when the outside pool is open. This time of year, I would opt for Palace.
If they are as potent as you guys all say...I may not survive two locations. 
Pace yourself. Take it slowly. Don't jump at the first girl who approaches. They all will eventually. Chat with the other guys. Have some food. Make a day of it. Bring enough money. Better to go home with a few extra Euros than with a regret not having spent time with the hottest babe in the place. When a chick strikes your fancy, spend a bit of time talking to get a sense of how enthusaiastic she is and what services she offers (prices are all fixed and posted on the board in the rooms). DO NOT BE AFRAID TO SAY NO.
Sounds like solid advice. Ever spend any time in the pools or saunas? I'm wondering if they are clean enough to go in with all the action that I hear from you guys takes place in these clubs.
Palace is too much of a minefield for the newb FKK hobbyist IMO. Moreover, the staff at Oase has been consistent over the years. While Palace has the location, Oase is better run and is much more custo friendly.
Interesting answer, rural, urban? They all seem like nightclubs from my view. While it depends when you visit, Palace has not outdoor area. Outdoors are a toss up when it comes to World and Oase. Both have pools and grassy areas with lawn chairs.
Positives
Oase has hands down the most women, interior is modeled after Caesars in Vegas (don't ask me how I know this) but note the ceiling.
World has hands down the large square footage
Palace is the closest to Frankfurt and airport
Negatives
Oase a bit hard to get to and find, small interior area when compared to the others. Alcohol is way too over priced 10 Euros a beer?
World mostly Eastern block, RO and BU gals, a bit too dark for my taste (lighting wise). You need to weed out the good from the bad women.
Palace, most expensive entry fee, known to have the most aggressive women with many operators. Majority of guests are non-locals which increases the chances of foreign STD's. While this is not limited to Palace it's more common. I've heard more people complain about this then any other club period.
While all clubs have good gals and bad, buyer beware. At one time women took pride in good service with the concept you would be back and become a regular. With the influx of new gals from RO and BU and tons of tourist traffic from conventions and the internet this has changed.
Just a week ago I was a Oase when I guy got taken for 400 Euros by two girls for 30 minutes of service. What makes this bad is when they get away with it they do it again and again. More girls figure it out and it becomes common place. Tourist often don't know the rules and prices and take it for granted the women don't lie. FACT: some do.
In the end, you'll have a good time at all three. My choice in order is
Oase
World
Palace
Pool and jacuzzi standards in Germany are rather high and believe it or not the health department checks them while they don't check the places you stick your tool with the same standards. Most clubs if not all forbid "sex in the water". At best you'll find a gal giving you a hand job but as far as mounting and blowing its highly unlikely. As far as hot, not many are as hot as I'd like and some are downright cool. The reason they often give for cool or cold pools is that people need cool water to immerse themselves after a hot sauna. Thus you also find tubs of cold water and barrels such as at World where one stands under the barrel and pulls the chain to dump the cold water over them. The colder the better. No need to worry about the sauna's the heat would kill pretty much most germs. If you do use them, remember you're suppose to bring a towel to sit on as to not allow sweat to touch the wood.