In the UK it was Margaret Thatcher that got the formerly Great Britain into selling arms for oil to the Saudis in a big and very corrupt way. From time to time Parliament has to exercise some serious levels of censorship to make sure that no details of this get discussed in the open.
https://en.wikipedia.org/wiki/Al-Yamamah_arms_deal
(Note the detail on 'sheep's eyeballs')
There are currently six thousand employees of British Aerospace (BAE Systems) out there working for the Saudis tinkering with their aeroplanes and keeping their air force at maximal readiness.
What does this entail?
There are no hard facts, any knowledge is strictly 'oral tradition'. However, I have it on reasonable authority (pub chat with people that work for the 'defence sector') that the Saudis have vast underground hangars chock full of the most modern BAE fighter jets. Because the Saudis have 'free oil' they also have these planes running in the hangars, absolutely totally ready to taxi out the door and onto the tarmac at a moments notice, 24/7, 365.25 days of the year.
The UK's RAF only has thirty thousand or so people in it, probably the bulk of them shuffle paperwork and are not front line. The UK Marines only number seven thousand. So this gives an idea of how large a commitment it is to have six thousand or so people from BAE Systems tinkering with planes in these Saudi bunkers.
Civilians in the UK have absolutely no idea about any of this. When they go to buy petrol they are thinking of what snacks to buy and why the other people in the forecourt are such chavs. They are not thinking about how they are indirectly complicit in the arms for oil (and backhanders) 'Al-Yamamah' deal. Or about how this has worked out for Iraq, Yemen or any of the other Saudi neighbours.
Anyway, it gets interesting soon for Saudi Arabia because they now have the Chinese customers wanting their own special favour. Thanks to The Donald the Chinese government want to buy oil in their own currency, the Yuan, bypassing the step of buying dollars first to then pay the Saudis for oil in USD.
Oil isn't the scarce reserve people think it is, supply has to be controlled so that profit can be made. If the Saudis don't want to take Chinese Yuan then they won't be supplying the largest importer of crude oil which is now China, not those fracking Americans. Iran will take the Yuan and I am sure Venezuela (who have the most proven reserves) will do to, given time and if the U.S. don't do a Libya on them. The Saudis certainly are living in interesting times.