AN ACR MD80 CAPT GOT ENG AND AIRFRAME ICING AT PLUS 13 DEG C.

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OAK WX XA45Z E 2500 FT OVCST; 7 MI RAIN SHOWERS; TEMP 56 DEGS; DEW POINT 56 DEGS; WIND 140 DEGS; 13 KTS; ALTIMETER 29.99 FT. DEPART TIME AT OAK WAS AT XB42 LCL. WX AT TKOF WAS VERY LIGHT RAIN; TEMP 60 DEGS; LIGHT S WINDS; RWY 11 IN USE; NOTHING TO INDICATE A NEED FOR ENG ANTI-ICE. TKOF AND CLB WERE NORMAL. AT APPROX 2000 FT; WE NOTICED SLEET ON THE WINDSHIELD AND BLADES; HOWEVER RAM AIR TEMP WAS READING PLUS 13 DEGS C WITH RAM AIR TEMP METER INDICATING NORMAL. ENGS AND INDICATIONS WERE ALWAYS NORMAL AND NO PROBS EVER ENCOUNTERED; HOWEVER ENG ANTI-ICE AND AIRFOIL WERE USED AS A PRECAUTION. HERE IS A CASE WHERE SLEET/ICE CAN BE PRESENT AT A HIGHER RAM AIR TEMP; WITH POSSIBLY A LOW ALT TEMP INVERSION/FRONT ALONG WITH A SLOW TO REACT OR STICKING RAT/THRUST RATING INDICATOR. A VISUAL OF ICE HAS HIGHER PRIORITY OVER RAM AIR TEMP AND WX BRIEF. FOOD FOR THOUGHT: CAN WE BE DAMAGING OR AT THE LEAST DECREASING THE LIFE OF OUR ENGS BY NOT USING ANTI-ICE MORE OFTEN IN SIMILAR SITS? MAYBE THE USE OF ANTI-ICE WITH RAM AIR TEMP 6 OR LESS WITH MOISTURE IS NOT CONSERVATIVE ENOUGH? DO OUR PROCS NEED CHANGING AS FAR AS RAM AIR TEMP IS CONCERNED; OR RAM AIR TEMP 6 DEGS; FRONTS; TEMP INVERSION; ETC. MANY MD80 CREWMEMBERS; MYSELF INCLUDED; HAVE EXPERIENCED AN ENG THAT SEEMED TO BE LAGGING/SLOWER TO RESPOND; WHEREBY THE USE OF MOMENTARY ENG ANTI-ICE HAS CORRECTED THE PROB. UNSEEN; UNEXPECTED ICE? CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR HAS HEARD NOTHING FROM HIS ACR EXCEPT AN ACKNOWLEDGEMENT OF HIS RPT. THE 'RAT' THAT HE REFERS TO IS RAM AIR TEMP; AND 'TRI' IS THRUST RATING INDICATOR.