No, it's never required. In some cases it might be a good idea.
General rule of them:
If you have over 100,000 miles, chances are there is some degree of sludge and varnish build up and would recommend one of two things:
1. Do an engine flush and then you're good to run the intervals the amsoil publishes.
2. You could just switch over without the flush but I would run my first two oil changes a little shorter interval, 7500 miles or so. The synthetic will be doing some cleaning so good to run it a little shorter. Then you'd be good to extend them after that.
It's a rule of them and pretty generalized. If you have over 100k and you've owned it the whole time and know it was taken very well care of, you probably don't have an issue. It's the trucks we buy used and don't know the history where I would do one these procedures if switching.
Heath -
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