In El inspector Morse (1987), Morse and Lewis worked for the Thames Valley Police - the actual police force that is responsible for this part of the country. However in Lewis (2006), Lewis and Hathaway work for the fictitious Oxfordshire Police Force because the producers were not allowed to use the name, uniforms, or emblems of Thames Valley Police and could not film at St Aldate's Police Station. It is thought that this is due either to Home Office security restrictions in connection with the Iraq war or because of a lack of cooperation by a newly appointed Chief Constable.
James Hathaway's father Phillip is introduced as a character in Endeavor Season 3 Episode 3 "Prey" as a young man. He is shown as a hand working on the estate and interacts with a young Inspector Morse and Detective Inspector Thursday. Arguably, Philip's character connects all 3 series in the one story arc.
The stories for Series 7-9 are each shown as two 45-min parts on consecutive weeks in the UK. This is in contrast to Series 1-6 stories, which are shown as single 90-min episodes. Therefore, Series 7-9 are filmed in blocks of two 45-min episodes. But confusingly, they were also repeated in the UK back in the more usual 90 minute version. This then brings back the series to last 33 episodes - the unofficial rule being that the spin-off(s) shouldn't last longer than the original "Morse".
Disregarding the two-part episodes, there are the exact same number of individual Lewis (2006) stories - 33 - as there was of its predecessor El inspector Morse (1987).
It took four weeks to film a 90-min episode. Only one of those weeks was spent in Oxford.