TM 55-2840-251-23 Section X   ENGINE TESTING (Continued) 1-93 GENERAL (Cont) CAUTION Do   not   mix   oil   of   different   military specifications, since the chemical structure of different brands or specifications    of    synthetic    oil    may differ sufficiently to make them incompatible with each other.  Should both types of oil become mixed, drain and    flush    oil    system    (Section    VIII, Paragraph 1-71) and refill with oil. g. Use lubricating Engine Fuel (E.19) and Lubricating Oil (E.2) or (E.71). 5.   The following test information is relevant: a. General.    The  performance  of  a  turbine  engine is greatly affected by surrounding atmospheric conditions.  For this reason, it is absolutely essential that accurate   barometric   pressure   (not   corrected   to   seal level)   and   ambient   temperature   are   obtained   for   the immediate   area   at   the   time   of   engine   trimming   and performance checks be carried out at low wind velocity with   the   engine   intake   facing   into   wind.      Even   this precaution will not prevent an unstable trim at moderate- to-high wind velocities. b. Precipitation.      Performance   and   trim   checks should   not   be   attempted   during   moderate   to   heavy precipitation or fog. Water   entering   the   engine   inlet   changes   the   power output  of  the  engine,  the  change  being  proportional  to the amount of water ingested.  Any form of moisture in the air causes the engine to ingest water.  This includes the occasions when water or snow is blown into the air inlet  by  the  propeller  or  sucked  in  by  the  compressor. Freezing   rain   or   slushy   snow   has   exactly   the   same effect   on   power   output   as   water   ingestion,   and   in addition may cling to inlet ducts or the inlet screen and upset the flow of air to the engine. c. Fuel.  All trims must be made using the normal Engine Fuel (E.19) used in servicing. d. Ground  Operating  Limits.    The  limits  shown  in tables 1.9.1 and 1.9.2 are provided as a guide and are based on sea level pressure and temperatures as specif ied. e. Engine     Overtemperature.          Overtemperature conditions are usually preceded by an excessively rapid rise  in  fuel  flow,  compressor  speed,  and/or  interturbine temperature. Several momentary high overtemperatures    affect    engine    service    life    just    as seriously  as  a  single  prolonged  lower  overtemperature condition.      The   higher   the   temperature,   the   sooner serious  engine  damage  occurs  and  the  more  extensive is the inspection required.  When an GO TO NEXT PAGE 1-137