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What weight Schaeffer oil are you running? From reading the posts here, 5W40 seems to be the right one but the guy selling it seemed aghast that I'd put anything other than 15W40 in my 7.3.
The 9000 is only available in a 5W40. That's all I run, all year round. I've been running 5W40 for the past 3 years like that. Did he say he why he thought that way? I'd hope he has some hard data and not just "I was told only to reccomend 15W40 for the 7.3."