Technically, neitherGulliver's Travels nor Oroonoko uses empirical evidence, because they are both works of fiction . Empirical evidence is gathered through direct observation. The lands to which Gulliver travels in his narrative are fictitious (as is the character of Lemuel Gulliver); therefore, no true empirical evidence can be provided...
inGulliver's Travels. This is a more relevant question with regard to Oroonoko because Suriname is a real country that was governed by the British and given to the Dutch. However, there is significant doubt as to whether Aphra Behn really lived in Suriname for a time.
Oroonoko was written in the style of a biography. At the time Oroonoko was written, the novel was still a relatively new form of writing in England. Using the style of a biography as recorded in a diary would likely have made the novel more accessible to readers. Gulliver's Travels, on the other hand, was specifically written in the style of a travelogue because of the popularity of the style at the time. Considering that both novels were written in a nonfiction style, we could consider "empirical evidence" to include things which would be empirical were the stories true. Under this approach, both novels have a significant amount of empirical evidence.
In Oroonoko, the majority of the novel would fall outside of the category of empirical evidence because it relates a story that was not directly observed by the author. Empirical evidence begins to appear once the author meets Oroonoko, though some scenes (the reunion of Oroonoko and Imoinda, the death of Imoinda, the rallying of the slaves, etc.) would not include empirical evidence, because they are built on information related to the author by others.
Gulliver's Travels is more difficult to separate into empirical and non-empirical sections, because the novel itself revolves around the direct observations of Lemuel Gulliver. Certain specific sequences, such as Gulliver's attempt to derive the origin of the name of Laputa, are deductive in nature rather than empirical. Additionally, Gulliver's conclusions as to the superiority of the Houyhnhnms and his conclusions as to the moral values of others are deductive rather than empirical. However, Gulliver's descriptions of the lands, cultures, and people themselves are all empirical in nature.
The use of empirical evidence on the narratives could have a number of effects. One effect that could be discussed is how empirical evidence can strengthen satire by more clearly identifying the subjects of scorn. The effect on readers could also be discussed, as the use of empirical evidence may provide readers with a different experience. A final potential route of discussion is whether it is possible to write a novel from the first-person perspective without including empirical statements.