* Crude futures end mixed as August Brent contract expires * Brent-U.S. crude spread seesaws, ends wider NEW YORK, July 16 (Reuters) - Most cash crude differentialsin the United States strengthened on Tuesday as thetransatlantic spread seesawed and then ended wider, traders andbrokers said. Crude futures were mixed, with Brent edging higher ahead ofthe August contract's expiration at the end of the session. Brent crude's premium to U.S. crude ended at$3.40 a barrel based on August contract settlements, wider thanthe Monday's spread of $2.77. The trade on Tuesday continued the recent trend of seesawingin a fairly wide range. Tuesday's range was from $2.32 to $3.73,after the spread ranged on Monday from $2.59 to $3.46. Friday'srange was from $2.41 to $3.36. The spread last Thursday shrank to $1.32 intraday - thenarrowest between front-month contracts since November 2010. On Tuesday, the spread between Brent and U.S. crudeSeptember contracts ended at $2.45 based on settlements. Usually, the wider the arbitrage, the more supportive it isfor U.S. cash crude differentials, while a narrower spread oftenpressures differentials. This especially holds true for sweetgrades, which are priced in line with global waterborne crudessuch as Brent. CASH CRUDE TRADES Light Louisiana sweet crude oil for August deliverytraded on Tuesday at $5.95 over the benchmark U.S. August crudefutures. That was stronger after Monday's trades from$5.35 to $5.70 over the benchmark. A trade for September LLS barrels was completed at $4.80over the futures benchmark. Mars sour crude, a Gulf of Mexico-produced grade,traded from 60 cents under to 85 cents under the futuresbenchmark, stronger after trading on Monday at 82 cents and 85cents and $1.00 under the benchmark. Trades for September barrels were completed at 90 cents and$1.10 below the benchmark. Bonito crude traded at $4.45 and $4.60 over thebenchmark for August barrels, well above the $2.90 over, wherebuyers' offers were pegged on Monday. Southern Green Canyon crude traded at $1.80 under thefutures benchmark, weaker from Monday's $1.45, $1.55 and $1.65under the benchmark. Bakken crude at the Clearbrook, Minnesota, hub tradedat $3.00 under the futures benchmark, where offers were peggedon Monday, but stronger than the bids seen at $4.00 under. MIDLAND GRADES West Texas Intermediate crude at Midland traded at 35cents under the futures benchmark, after trading on Monday at 40and 35 cents under the benchmark. West Texas Sour crude, also at Midland, traded at 25cents under the futures benchmark, after trading on Monday 25and 20 cents under the benchmark. CRUDE FUTURES The expiring Brent August crude contract rose 31cents, or 0.28 percent, to settle and go off the board at$109.40 a barrel. Brent September crude rose only 6 cents to settle at$108.14 a barrel. U.S. August crude fell 32 cents to settle at $106.00a barrel. The U.S. August contract expires on July 22. (Reporting by Robert Gibbons, editing by G Crosse)