98 RON is a Australian octane measurement - in Canada, the United States, Brazil, and some other countries, the headline number is the average of the RON (reference octane number)and the MON (motor octane number), called the Anti-Knock Index (AKI), and often written on pumps as (R+M)/2. It may also sometimes be called the Posted Octane Number (PON). this gives a number lower than the RON alone. The US trucks had lower compression motors, so 87 octane was fine for these trucks when new.