The MIL-L-23699 has a higher flashpoint and higher viscosity at any given temperatures than MIL-L-7808. MIL-L-23699 can be used in temps down to -40 deg's whereas MIL-L-7808 can be used down to -54. I operate the T-56-14 in P-3C aircraft, which is the same engine as the Herc and all of our aircraft utilize MIL-L-23699. It really depends on your operating area and the climate of that area. Our ceiling is 32,000 ft and we've operated out of various cold areas, never had a problem with the 23699. Highly recommend the 23699, however, when and if you decide to convert, DO NOT MIX THEM! Drain/flush the 7808, including filters and run the 23699 through the system for 20 flight hours, then change it. You should be good to go from there. Have fun and keep'm on centerline!