I prefer to oil my decks, you don't have too but it does offer a good level of protection and makes the timber look great, the best oil is certainly Owatrol and D1 or perhaps D1 Pro.
I would have a chat with Thomas here http://deckingnetwork.com/profile/ThomasRathbone a decking renovation specialist.
Sorry its taken a while to reply to your question, been frantically busy, This is very much a personal thing and will come down to what the client wants. What you will find with Ipe, is that it will weather very quickly, starting to lose its colour within a couple of months especially if the deck is south facing and exposed. If the client wishes to keep the fresh look of the timber then I would most certainly offer to oil it and for this I would choose D1 Pro as opposed to D1.
D1 Pro was specially developed for exotic hardwoods such as Ipe, Cumaru etc which are dense and difficult to penetrate. It has the penetrating qualities of D1 but with added UV filters, which not only help to keep the wood looking new longer but also enhance the natural reddish brown of woods such as Ipe. If oiling is chosen by the client then preparation of the wood is important to get the best from the finish and for this I would use Prepdeck followed by Net-trol. Due to the density of Ipe it is now suggested that new Ipe is treated with a 1 to 1 Prepdeck / water solution as opposed to the normal 1 to 4 mix. Using Prepdeck will have several benefits - it will remove any mill glaze, open the grain of the wood, wash out excess oils and degrease the surface of the wood - thus helping the oil to penetrate better and more easily. This should all be followed with Net-trol diluted 1 to 1 with fresh water to restore the woods colour - Prepdeck reacts with the tannins in the wood causing it to darken. Once preparation is complete and the wood allowed to dry for a couple of days then application of the oil can take place.
I would not recommend sanding the wood and then oiling. The issue with sanding is that while it may remove some of the mill glaze it will not fully open the pores of the Ipe especially if you do not use a coarse enough grit, nor will it remove any excess tannins or degrease the surface all of which will restrict the penetration of the D1 Pro into the Ipe.
I hope all of the above helps.
A very helpful and comprehensive post - Thank you very much!
It's not consistent, but here in the States (where ipe is wildly popular) we're hearing more and more contractors telling us they're sealing all four sides of the planks before installation. The thinking is that the planks won't be oiled again, and it helps protects against the ingress of moisture that could contribute to cupping. (the cut ends, of course, get a coating of a wax-based emulsion to slow or stop the end-checking)
Most installers do sand before application.
We've just put our first Ipe deck down and, with Amy's post above, found two boards starting to cup within days following the recent heat-wave we've had in the UK. We solved the problem by soaking the board (it then laid nearly flat), strapping the underside of the board (yes, it wasn't on a beam) and then screwing up into the cup ends to pull them down. Once this repair was effected, a little light sanding on one side and then oiling sealed the deck.
When we get our next project, I'm seriously considering Amy's advice of sealing all four sides pre laying.
Cupping is due mainly to 2 factors.
1. The way the board has been cut from the log, tangentially or flat sawn (99% of decking) is where you will see the most movement and 1/4 cut has almost no movement. The cost of 1/4 creates lots of waste and is very, very expensive.
2. The equilibrium of moisture on the upper side of the board should be the same as the underside. If you have a wet underneath and a dry top as the moisture transpirates out of the timber it will cause movement in that side, so if the outer side has no movement then the board will cup.