Canadian oils weakened ahead of planned refinery maintenance later this year that will curb demand for the grades.

BP Plc (BP/)’s Whiting refinery in Indiana will shut a crude unit and several compressors next month for six to eight weeks of work, a person with knowledge of operations said July 10. Marathon Petroleum Corp. will shut its Detroit refinery for maintenance by Sept. 6, a person with direct knowledge of the plans said March 1.

Syncrude weakened $6.50 to a discount of $3.50 versus West Texas Intermediate at 12:08 p.m. in New York, according to data compiled by Bloomberg. Syncrude is a synthetic oil upgraded from tar-like bitumen in Alberta into refinery-ready crude.

Western Canada Select’s discount widened $1 to $20 below WTI. Bakken oil’s discount to WTI weakened $1 to $4 a barrel.

On the U.S. Gulf Coast, sweet oil premiums narrowed against WTI. Heavy Louisiana Sweet’s premium to the U.S. benchmark slipped 75 cents to $15.75 a barrel. Light Louisiana Sweet’s premium to WTI lost $1 to $16.50.

Poseidon’s premium added 80 cents to $10.05 a barrel. Southern Green Canyon rose 50 cents to $9.50 over WTI. Mars Blend increased 80 cents to $11.05 a barrel over the U.S. benchmark.

The premium for Thunder Horse, a sour crude with lower sulfur content than Mars, Poseidon and Southern Green Canyon, increased $1.45 a barrel to $12.75.

To contact the reporter on this story: Aaron Clark in New York at aclark27@bloomberg.net

To contact the editor responsible for this story: Bill Banker at bbanker@bloomberg.net