AN ACR B737-800 FLC RPTS THAT AS HE TURNED ONTO RWY 8; DURING A LIGHT RAIN; THE ACFT SKIDDED ON THE WET SURFACE; BUT STOPPED BEFORE LEAVING THE RWY.

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WX IN AREA WAS RAIN SHOWERS; AND IT HAD BEEN RAINING LIGHTLY FOR ABOUT 1 HR. I WAS CLRED FOR TKOF WHILE ON THE PARALLEL TXWY. I TAXIED ONTO THE RWY WITH SLIGHTLY MORE THAN MINIMUM PWR. AT APPROX A 45 DEG ANGLE TO THE RWY CTRLINE; AS I REACHED THE RWY CTRLINE AND TURNED THE NOSEWHEEL HANDLE I RECEIVED NO RESPONSE; AND THE ACFT CONTINUED AT THE 45 DEG ANGLE TOWARD THE EDGE OF THE RWY. I APPLIED MAX BRAKING AND COULD FEEL THE ACFT SKIDDING AS THE ANTI-SKID OPERATED. THE ACFT CONTINUED TRAVELING TOWARD THE SIDE OF THE RWY; AND I APPLIED REVERSE THRUST TO THE ENGS. THE ACFT WAS SKIDDING AND SHUDDERING; BUT STOPPED ON THE RWY APPROX 10-15 FT FROM THE EDGE. I BROUGHT THE ENGS TO IDLE; AND THEN SLOWLY CORRECTED THE ACFT TO THE RWY CTRLINE; AND A NORMAL TKOF AND DEP ENSUED. THE RWY WAS ASPHALT; AND I DO NOT THINK IT IS GROOVED. THE RAIN; IN COMBINATION WITH OILS FROM THE ASPHALT AND ACFT; TIRE; RUBBER; MADE A NORMAL TAXI ONTO RWY FOR TKOF A VERY SLIPPERY SIT. IT HAS MADE ME VERY WARY OF ALL WET RWY AND TXWY TAXI/TKOF/LNDG EVOLUTIONS. DURING THE WINTER I TEND TO FOCUS ON SNOW AND ICE; BUT THIS SIT OCCURS YR ROUND; AND COULD PUT YOU OFF A RWY OR TXWY WHEN YOU ARE NOT EXPECTING TO SLIP. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR SAID THAT HE OPERATES INTO SJU FAIRLY OFTEN; BUT THIS WAS THE FIRST TIME THAT HE HAD ENCOUNTERED A SLIPPERY RWY CONDITION THERE. HE HAS SINCE STUDIED THE RWY SURFACE; AND ALTHOUGH IT IS GROOVED; IN THIS AREA ON THE TKOF END OF RWY 8 THE GROOVES APPEAR TO BE FILLED WITH RUBBER; GREASE; OR OTHER DARK COLORED DEBRIS THAT; WITH A COATING OF WATER; MADE THE SURFACE VERY SLIPPERY. HE SAID THAT HE NOTICED THAT THE PAINTED LINES ON THE RWY SURFACE WERE PARTIALLY OBSCURED BY THIS COATING. THE RWY IS CROWNED; AND DRY TRACTION IN THIS AREA IS GOOD. THE CAPT DID NOT MENTION THIS INCIDENT TO THE TWR CTLR.