The G-series engine seems to be the engine to have but will be in very small quantities. F-series seems to be one step below the G-series, and E1-series is the absolute minimum. E0 engines were the ones that were recalled (so if any still left out, stay well clear).
A guy called James on PH posted the engine saga chronology last August, so many thanks to him (I hope he doesn't mind me reposting it) ...
The early GT3 cars, built up to May 14, had the original spec of MA175 engines with the prefix 'E0' (so: E01234 for example) where a handful (only) of them exhibited the conrod bolt issue (thought to be caused by the bolts not holding torque correctly, leading to the rod coming loose, punching a hole in the block, causing oil egress and the possibility of a fire). These early vehicles were subject to the 'stop sale/stop driving' recall in early 2014 and were subsequenty fitted with replacement engines. There were around 765 of these original 'E0' engines replaced, either at the factory (post May 14 as above) where cars were yet to be delivered, or at the supplying dealer where the car had been supplied already.
The replacement engines (and, indeed, engines fitted in series production for the remainder of MY14) were effectively to an updated MY14 specification which carried the prefix 'E1' (so: E11234 for example). They were built wth an 'optimised conrod bolt' but it is thought that there were also process changes in the factory to more accurately control the torquing of the bolts during engine assembly. I don't think there have been any reported incidences of any 'E1' (or later) spec engines exhibiting the conrod bolt issue.
However, as these 'E1' spec engines were used and increasingly subjected to in-service operations, including track work, a small proportion of them began to exhibit a top-end wear problem (wear on the rocker fingers/followers and/or on cam lobes). This was signalled to the driver by the illumination of a 'reduced performance' warning light, itself triggered by the wear in the upper valvetrain area causing a small timing issue at high engine rpm (typically 8500 rpm and above). Although the incidence of these failures is believed to be small, it appears that the root cause was a combination of insufficient lubrication in these areas combined with an incorrect or insufficient depth of the very hard 'diamond-like coating' (DLC) on the interfacing surfaces. It is likely that this design or process deficiency was also present in the original, pre-replacement engines but never came to light because of their relatively short in-service period.
In instances where these 'E1' spec engines have failed, they have typically been replaced with a new unit - generally being to the MY15 'G' spec engine (see two paragraphs below). It is these cars, believed to be very small in number, which produce the stories about cars having two replacement engines; the original replacement to overcome the conrod bolt issue and a second change to address the upper-valvetrain lubrication/wear problem.
In parallel to these issues becoming known on the MY14/'E1' spec engines, Porsche updated the engine again with a series of running-changes which were introduced at the start of MY15. These engines carry an 'F' prefix. The modifications are thought to include changes to address the emerging lubrication issue (incluidng uprated camshafts and followers/fingers) and some new ECU parameters to modify oil pressures under specific conditions. I think that the number of reported issues with valvetrain wear on 'F' spec engines is very low, although more time in-service is required to give complete confidence.
Finally, for MY16 (yes, there were a few cars...) a 'G' specification of engine was introduced. This went further than the mods introduced in the 'F' spec and incorporated additional engineering changes that had been developed for the MA176 RS engine, including a new oil pump and filter arrangement.
It is thought that, where 'E1' or 'F' engines show evidence of the lubrication and valvetrain wear problem described above, these units will be replaced with the latest 'G' spec (RS based) engine design, and that this approach forms the basis of the 10-year engine warranty that Porsche has spoken about (and which is expected to be formally confirmed by Porsche GB in the coming weeks).