I have been having a hard time finding oil that meets the GF-3 requirement, but can always find GF-4 oil. I did not know if the GF-4 was safe to use in the Rx8's engine or if i need to keep looking for GF-3.

 

ILSAC GF-3 an oil must meet both API SL and the EC-II requirements. The GF-3 standard has more stringent parameters regarding long-term effects of the oil on the vehicle emission system, improved fuel economy and improved volatility, deposit control and viscosity performance. The standard also requires less additive degradation and reduced oil consumption rates over the service life of the oil.

The ILSAC GF-4 standard was introduced in 2004 and increases the compliance parameters over GF-3 by a significant margin. Furthermore GF-4 oils are backwards compatible and may be used wherever GF-3 or API SL was specified.

 

what about GF-5 oil?