1999 Lexus GS 300 Owners Manual - Page 407

ENGINE FUEL Fuel type Your vehicle must use only unleaded gasoline. To help prevent gas station mixups, your Lexus has a smaller fuel tank opening. The special nozzle on pumps with unleaded fuel will fit it, but the larger nozzle on pumps with leaded gas will not. At a minimum, the gasoline you use should meet the specifications of ASTM D4814 in the U.S.A. and CGSB 3.5-M93 in Canada. Use of unleaded fuel with an octane number or rating lower than stated above will cause persistent heavy knocking. If severe, this will lead to engine damage. If your engine knocks ...If you detect heavy knocking even when using the recommended fuel, or if you hear steady knocking while holding a steady speed on level roads, consult your Lexus dealer. However, now and then, you may notice light knocking for a short time while accelerating or driving up hills. This is no cause for concern. Gasolines containing detergent additives NOTICE Do not use leaded gasoline. Use of leaded gasoline will cause the three-way catalytic converter to lose its effectiveness and the emission control system to function improperly. Also, this can increase maintenance costs. Lexus recommends use of gasolines that contain detergent additives to avoid build-up of engine deposits. For further details, ask your Lexus dealer or a local gasoline retailer. Improved gasolines The American Automobile Manufacturers Association (AAMA) has developed a specification of improved gasolines. The AAMA specification offers optimal fuel information for better vehicle performance and better protection of your engine. Lexus recommends the use of gasolines that meet the AAMA specification, if available, for improved drivability 387 Octane rating Select premium unleaded gasoline with an Octane Rating of 91 (Research Octane Number 96) or higher for optimum engine performance. However, if such premium type cannot be obtained, you may temporarily use unleaded gasoline with an Octane Rating as low as 87 (Research Octane Number 91).