By Mark Lamport-Stokes

OAKMONT, Pennsylvania (Reuters) - Phil Mickelson heads into this week's U.S. Open at Oakmont Country Club hoping that the daunting par-70 layout plays as difficult as possible in his pursuit of a career grand slam of the four majors.

The American left-hander, a perennial fan favorite, has been a runner-up in his national championship a record six times without ever winning but feels he can only benefit from his 25 previous U.S. Open starts if the course goes "over the edge".

Should Mickelson go on to triumph at Oakmont this week, he would become at 46 the oldest U.S. Open champion and just the sixth player to complete a grand slam of all four major professional crowns.

"After 25 years, you have to really know how to play this style of golf," Mickelson told a packed news conference on Wednesday before setting off for a practice round at Oakmont. "It's not like going out and playing at any other golf course.

"This is a whole different style of golf, something that over the years I've become very effective at playing. Because of that, I would love to see it cross the line the way U.S. Opens often do, and become a little bit over the edge.

"I feel like I've learned how to play that style of golf, and this golf course, specifically ... I feel like I've developed a game plan now coming in that will allow me to shoot the lowest score. But you still have to execute."

Mickelson, a five-times major winner, said that a first U.S. Open success would cap off what is already a Hall of Fame career but he is wary of allowing desired results to affect his performance during the week.

"This is the tournament I want to win the most to complete the four majors, there's no question," said the American left-hander. "But I have to put that out of my head and try to execute and be patient, not think about results.

"You start thinking about results, you'll never play your best golf ... but there's no question that starting this year and every year here forward until I ultimately win this tournament, it will be my biggest thought, my biggest focus.

"I view those players that have won the four majors totally different than I view all the others."

Victory at Oakmont on Sunday would allow Mickelson to bury memories of his multiple near-misses at the U.S. Open and put his name alongside Gene Sarazen, Ben Hogan, Gary Player, Jack Nicklaus and Tiger Woods on the career grand slam list.

Mickelson's preparations for the year's second major have been a little unusual this week as he flew back to California on Monday night to attend his daughter Sophia's eighth-grade graduation in San Diego on Tuesday.

"I think it's a very important thing in a person's life that I was fortunate enough to be able to be at," said Mickelson. "It's just important for me to be there for that stuff."

Mickelson was asked how stressful the last couple of months had been for him before he was named as a "relief defendant" last month in a civil lawsuit by the U.S. Securities and Exchange Commission.

The golfer was not accused of wrongdoing but had received ill-gotten gains as a result of others' illegal acts and he agreed to pay back $1.03 million, including profit and interest, to resolve claims from his role in the scheme.

"I feel like I'm playing stress-free and much better golf. That might have something to do with it. I don't know. But I'm excited that it's behind me," said Mickelson.

(Editing by Frank Pingue)