The above picture shows the injector sleeve removed from a 4JX1 engine. The sleeve has 2 Orings which separate the oil, fuel and coolant flowing around sleeve. Some problems have resulted from an O ring deteriorating thus leading to oil and fuel or coolant and fuel mixing. Some Aussie sites suggest replacement of these Orings before 80k kms.
This pic shows the injector assembly. Each injector has 2 Orings that keep separate the oil and fuel.
Based on discussions with casa mechanics, it is important to keep the fuel free from any water and the coolant changed regularly.