A new shortage of synthetic motor oil has retailers imposing new limits on purchasing.Costco this week raised the prices of its store brand synthetic oil and placed maximum limits on synthetic oil purchases.A two-pack of Kirklan d Signature full-synthetic motor oil 5-quart bottles is now $58, up from about $30 the same time last year. Customers will be limited to two boxes of oil per week at Costco stores.RELATED STORY | Oil prices leap to their highest since May and drag Wall Street lowerThe shortage began in May as refineries put their focus on producing more gasoline.Most synthetic motor oil is derived from the crude oil refinement process. The U.S. imports more than 40% of its supply from the Middle East, where the flow of oil has been restricted due to the ongoing conflict in Iran.The cost of a barrel of oil pushed above $109 this week, and the cost of diesel fuel set another new record following last week's all-time high.MORE ON ENERGY COSTS | Diesel prices set a new record as Trump calls on Ukraine to stop targeting Russian oilPresident Donald Trump acknowledged Wednesday that gas prices were a little higher due to the war his administration began in Iran, but he said those prices are "a very inexpensive price to pay for what we've done.""Even if it was a lot more," he said, the price would be worth preventing Iran from obtaining nuclear weapons capability. ...read more read less