SINGAPORE, Sept 13 (Reuters) - Asian gas oil's front-month timespread climbed to a 4-month peak on Tuesday, buoyed by strong buying interest in the physical spot market and signs of firmer demand in the region. September/October rose 14 cents to 30 cents a barrel, its highest since May 10 by 0830 GMT, Reuters data showed. However, the October/November timespread slipped further in contango to minus 4 cents, down 2 cents. However, the prompt October crack fell by almost a dollar to below $17 a barrel, partly depressed by higher crude prices overnight. No physical trades were concluded in the pricing window, although aggressive bids by Hin Leong helped push the 0.5 percent sulphur gas oil grade's cash differential to parity, a 3-1/2 month high. The Singapore trader has bid up the differential and accumulated over 1.4 million barrels of the high-sulphur grade this month. Fundamentals remained steady, with higher demand from Indonesia offset by high supplies in the region. While stocks of middle distillates in Singapore and Japan eased last week, the gradual return of Formosa Petrochemical Corp.'s refinery in Taiwan will add to stockpiles in the coming months. A Formosa spokesman said on Thursday its 540,000 barrels per day refinery is running at 70 percent capacity, after being completely shut end-July following a fire at a propylene recovery unit which destroyed power cables. "The refinery is now running at around 400,000-plus bpd after we restarted the last of three crude distillation unit (CDU) last week," the spokesman said. "We cannot run at maximum rates as we have yet to restart one of our two residual fluid catalytic crackers (RFCC)." The RFCC is to restart around Sept. 20, he added. In the swaps market, at least 450,000 barrels of the gas oil September swap were done at $122.60 a barrel, up from 300,000 barrels a day earlier. Another 150,000 barrels of the September/October timespread changed hands at 25 cents a barrel. * SWAPS OUTRIGHTS: Gas oil's September swap rose 60 cents to $122.50 a barrel, while the October swap gained by 46 cents to $122.20 a barrel. - Jet fuel swaps for September added 60 cents to $123.40, while the September regrade, or the difference between jet and diesel prices, was steady at 90 cents. * CRACKS: Gas oil's crack for October fell 99 cents to $16.34 a barrel over Dubai crude. - Jet fuel's October crack lost 84 cents to $17.99 a barrel. * CASH DIFFERENTIALS: The discount for gas oil with 0.5 percent sulphur narrowed 5 cents to be valued at parity, while the premium for the 0.25 percent sulphur grade gained 5 cents to 85 cents. The 0.05 percent sulphur grade also rose 5 cents to a premium of $1.95. - Jet fuel's cash discount added 10 cents to 55 cents. * PHYSICAL OUTRIGHTS: Benchmark diesel with a maximum sulphur content of 0.5 percent was 60 cents higher at $122.35 a barrel, while jet fuel rose 35 cents to $122.75. * CASH DEALS: No deals reported. PRODUCT RIC BID ASK MEAN PREV CHG % CHG Jet/Kero JET-SIN 122.55 122.95 122.7 122.4 +0.35 0.29 5 0 Gas Oil 0.5% GO-SIN 122.15 122.55 122.3 121.7 +0.60 0.49 5 5 Gas Oil GO25-SIN 123.00 123.40 123.2 122.6 +0.60 0.49 0.25% 0 0 Gas Oil GO005-SIN 124.10 124.50 124.3 123.7 +0.60 0.49 0.05% 0 0 Jet/Kero JET-SIN-DIF -0.65 -0.45 -0.55 -0.65 +0.10 n.a. Diff GO 0.05 Diff GO005-SIN-DI 1.85 2.05 1.95 1.90 +0.05 +2.63 F GO 0.25 Diff GO25-SIN-DIF 0.65 0.95 0.80 0.80 0.00 0.00 GO 0.5 Diff GO-SIN-DIF -0.10 0.10 0.00 -0.05 +0.05 n.a. GO Swap M1 GOSGSWMc1 122.5 121.9 +0.60 +0.49 0 0 GO Swap M2 GOSGSWMc2 122.2 121.7 +0.46 +0.38 0 4 (Reporting by Francis Kan; editing by William Hardy)

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