LONDON, March 18 (Reuters) - Gasoline cracks in northwest Europe weakened on Tuesday, hit by dwindling demand for winter grade product as the market prepares for the switch to pricier summer blends.     Exports to the United States and West Africa were brisk, however, as buying interest for summer-grade gasoline picks up ahead of the seasonal peak, traders said.     In new fixtures, one tanker was booked to carry unleaded to West Africa, Reuters shipping data showed. Another tanker was scheduled for Mexico, the data showed.     In the Mediterranean, no cargoes traded in the price assessment window. Gasoline in the Mediterranean, where summer gasoline blends are already on the market, was trading at a steep premium to cargoes in northwest Europe where the barge market was still being priced in winter specifications.     "The north is very weak," said one trader.     Naphtha values had decreased in recent days due to lower demand from Asia but nevertheless found support from demand as blending stock into the gasoline pool.     Traders said Egypt had offered a cargo of naphtha loading from Alexandria on the Mediterranean.                 GASOLINE  
  EURO/DIFF/MOGAS
        * No barges of Eurobob traded in the Platts price assessment window and there were no bids or offers     * Some 12,000 tonnes traded on the Argus platform at $910-$911 a tonne fob ARA, down from $912-$928 a tonne on Monday.       * Total and Trafigura were on the sell side. Gunvor, Shell and Vitol bought.      * At the close, the April swap was trading at around $986 a tonne down from $1,003 a tonne on Monday.     * No barges of winter premium unleaded gasoline traded. There were discussions at $903-$935 a tonne fob ARA.      * By 1729 GMT, Eurobob's crack to dated Brent  
  BFO-
   was at around $1.29 a barrel, down from around $2.11 a barrel on Friday.     * Brent crude oil futures  
  LCOc1
   were up 40 cents at $106.64 a barrel.  
  O/R
       * U.S. RBOB gasoline futures in New York  
  RBc1
   were up 0.69 percent at $2.9009 a gallon, and the prompt crack  
  RB-CL1=R
   was weaker at $22.38 a barrel.          NAPHTHA  
  NAF-C-NWE
        * No cargoes traded in the price assessment window. There were three bids and five offers but no deal was reached.      * The April naphtha crack swap was at minus $6.30 a barrel.   (Reporting by Lin Noueihed; editing by Keiron Henderson)   ((For latest physical prices double click on:  
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  Keywords: MARKETS EUROPE/GASOLINE