Mustafa Abdullah, 34, of Stockwell, south London, claimed to be on the M15 payroll. He was caught with the material at Gatwick Airport when he came back from Syria last year.
Police found gun instruction videos and audio recordings about radical, violent and fundamentalist Islamic ideology.
The jury was shown images of Abdullah posing and holding firearms.
Abdullah, from Binfield Road, worked as an appliance engineer. He told jurors MI5 paid him Â£10,000 for helping them and said he only went to war-torn Syria to do humanitarian work.
At the Central Criminal Court, he denied 14 counts of possessing terrorism documents or records, and one of possessing a gun for a terrorism purposes.
After deliberating for 12 hours, the jury found him guilty of 13 offences of possession of documents likely to be useful to a person preparing an act of terrorism.
He was cleared of the remaining charges.
In 2007 Abdullah was convicted and jailed for two years for possessing a document useful to terrorism, namely a guide on how to be a jihadist.
A jihadi has been jailed for four-and-a-half years after police found he had a number of terror training videos.