Here, let's start a contraversy.
Synthetics last a lot longer than petroleum based oils. They just don't break down as quickly. The 3000 mile interval recommended by car manufacturers is for heavy stop-and-go driving like what a taxi cab or police car does (or heavy traffic in hot weather). 7500 miles is the interval recommended for regular driving. These intervals are also for petroleum based oils which will break down and can cause varnishing and sludge build up.
The biggest problem that synthetics have is getting dirty. All anybody using a good synthetic really may need to do is change the filter every 3 to 6 months depending on the filter. 3 for basic and 6 for that new Fram filter (They say it's good for 7000 miles). Change oil and filter at 1 year intervals. Let the debate begin, or not.
Autolite copper cored non-platinum 25s are a great, inexpensive plug.
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351W 89 Mustang GT Convertible