March 7 (Reuters) - U.S. cash crude grades firmed marginally on Friday, dealers said, steadying after a rollercoaster week of uncertainty in the market around U.S. tariffs on Mexico and Canada.

U.S. President Donald Trump imposed a 25% tariff on imports from Mexico and Canada, with an exception of a 10% levy on Canadian oil, which took effect overnight on Tuesday.

Then on Thursday Trump suspended the tariffs on most goods from Canada and Mexico, in what was the latest twist in a fluctuating trade policy that has whipsawed markets and fanned worries about inflation and growth.

Earlier this week, Mars Sour WTC-MRS had surged in one of the first indications that Trump's tariffs were disrupting the global oil trade.

The coastal grade, which is an attractive alternative to Mexican grade Isthmus, firmed 5 cents on Friday at a midpoint of a $1.65 premium after easing since Tuesday.

Light Louisiana Sweet WTC-LLS, which had also eased throughout this week after being bid up on Tuesday, also firmed slightly on Friday, to a midpoint of a $3.88 premium.

Amid tariff news, dealers also will need to focus on Russia supply, after Trump said in a post on Truth Social that he was "strongly considering" sanctions on Russian banks and tariffs on Russian products because its armed forces continue attacks in Ukraine.

An indication of future oil production, U.S. energy firms this week cut the number of oil and natural gas rigs operating for the first time in six weeks, energy services firm Baker Hughes BKR.O said on Friday.

* Light Louisiana Sweet WTC-LLS for April delivery firmed 13 cents at a midpoint of a $3.88 premium and was seen bid and offered between a $3.50 and $4.25 a barrel premium to U.S. crude futures CLc1

* Mars Sour WTC-MRS firmed 5 cents at a midpoint of a $1.65 premium and was seen bid and offered between a $1.55 and $1.75 a barrel premium to U.S. crude futures CLc1

* WTI Midland WTC-WTM firmed 5 cents at a midpoint of a $1.20 premium and was seen bid and offered between a $1.10 and $1.30 a barrel premium to U.S. crude futures CLc1

* West Texas Sour WTC-WTS firmed 3 cents at a midpoint of a 83-cent premium and was seen bid and offered between a 70-cent and 95-cent a barrel premium to U.S. crude futures CLc1

* WTI at East Houston WTC-MEH, also known as MEH, traded between a $1.35 and $1.55 a barrel premium to U.S. crude futures CLc1

* ICE Brent May futures LCOc1 rose 90 cents to settle at $70.36 a barrel

* WTI April crude CLc1 futures rose 68 cents to settle at $67.04 a barrel

* The Brent/WTI spread WTCLc1-LCOc1 widened to last trade at minus $3.60, after hitting a high of minus $3.48 and a low of minus $3.61.

(Reporting by Stephanie Kelly in New York; Editing by David Gregorio and Alan Barona)

((Stephanie.Kelly@tr.com))