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It probably got the oil hot and thinned out. An oil cooler would help, but there ain’t but so much you can do about the power. Were you letting it gear down and rev high? Some people think they’re hurting an engine by revving it hard and they want to lug it but a gas engine isn’t going to make power down low. With my GM 6.0 and current 8.1 truck it’s very normal for me to pull a grade at 4,000+ RPM.