US cash crude-Grades firm, futures drop after stock build

* Grades firm as contango spread widens

* US oil futures fall on inventory gains, snap 6-day rise

NEW YORK, April 7 (Reuters) - U.S. cash crudes firmed on Wednesday as contango spreads widened and oil futures fell, snapping six days of gains, following data that showed U.S. crude stocks rose for a 10th straight week last week.

Light Louisiana Sweet LLS- firmed up to 40 cents to trade for $1.80 over West Texas Intermediate CLc1. Mars sour MRS- strengthened up to 50 cents to trade for $3.25 a barrel below WTI.

For a list of other cash crude deals see [CRU/TU]

On futures markets, May WTI CLK0 fell $1.16 a barrel after settlement to $85.68 a barrel. May Brent LCOK0 fell 84 cents to $85.31. [O/R] Futures dropped after six straight days of gains, following Energy Information Administration data on Wednesday which showed U.S. crude stockpiles continued to expand last week.

But cash crudes gained force after EIA data also showed gasoline stockpiles fell sharply last week and refinery utilization increased. [EIA/S]

The trans-Atlantic arbitrage CL-LCO1=R narrowed to 39 cents from 58 cents in favor of WTI after market settlement. The May-June WTI spread CL-1=R was 62 cents, up from 39 cents on Tuesday.

The last publicly disclosed sale of Alaska North Slope crude oil ASW- for May delivery was done last week at $3.25 a barrel under WTI.

California buyers reduced posted prices by 95 cents a barrel. (Reporting by Joshua Schneyer)