You can use unleaded gas in this car. Ferrari had been using for a few years before this car, cast-iron valve seats and this one features it too. Cast-iron is hard enough that it does not wear down like softer materials used in much older engines (like bronze) and does not need lead to maintain its shape and condition.
1. Does the 308 GTB (carburated) and the 308 GT4 have the same engine -- so I can use the 308 GT4 workshop manual with no problem?
2. What mileage should I change the tensioner bearings?
Yes, they are the same engine and there's only one Workshop Manual for the carb 308's. It's titled as a 308 GT4 manual, but sub-titled: "Also covers 1976-79 308 GTB/GTS models".
Tensioner bearings should automatically be changed when installing new timing belts. You must check your car's service history to see when this was done, but to be on the safe side, belts and tensioners every 4-5 years or 30,000 miles...
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