I just finished running a one-off game (over half a dozen sessions) using the D6 Adventure system. Though I've played (and run) plenty of Star Wars D6 in the past, this was the first time I've tried using the D6 system for another setting.

Published by West End Games in 1996, D6 Adventure (along with D6 Fantasy and D6 Space) is a set of rules tailored for a specific genre. Though they share a lot of the core mechanics (roll a number of D6 equal to your skill, try and beat a target number), there are some differences in terms of the names given to attributes and skills. The Adventure rules are aimed at 'contemporary' settings - sometime around the 20th century.

My own game was based on the idea that the PCs were members of German military intelligence (the Abwehr) in 1927 (so prior to Hitler grabbing power, and at the height of the roaring twenties), hunting down a rogue agent who was selling secrets to French communists. It was relatively pulp, and wasn't entirely based in the real world since they soon found some glass tablets that allowed them to see a live map of the surrounding area (including being able to see people moving around, or zooming out to see the whole Earth). A chase from France to Russia and finally Sumatra led them to discovering a way to a post nuclear-war future, and evidence that they were living in a fake world running in a simulation (though they didn't quite understand that part, being from the 1920s). We ended the game with them getting hold of nuclear weapons, and planning a way to ensure Germany didn't lose the next war.

A few things I found useful were Youtube videos of the time period. Since most people were unfamiliar with the period (myself included), it was helpful to stick some videos of the various cities up on the TV (thanks to Chromecast) when they got there, so they had an idea of what they were seeing. I also shared pictures of people and artefacts in this way.

For the system, I had some difficulty balancing the combat system. Players were a lot more effective than I first assumed, so I had to scale up combats a bit towards the end to make them a bit more challenging. I played it far more narrative than I normally would though, so it wasn't too bad. There were some options though we had to 'fix' pretty early on though (+3 to difficulty to get a head shot doing +12 damage, when most guns were rolling 3D6 for damage felt very broken) in order to make things a bit more interesting.

When 2nd edition Star Wars came out, and introduced the wild die, my natural inclination at the time was to ignore it. We tried using it in this game, and personally I wouldn't use the wild die again. Rolling multiple D6 is already sufficiently random that it doesn't need any more, and you lose the consistency of high skill rolls that you got in Star Wars (there's still a chance of rolling 6 1s, but the chance of a really bad (or really good) roll goes down as your skill goes up).

On the whole though the system worked reasonably well. I chose it because I knew the base system, and leaving out the advantages/disadvantages options left a really simple system that was quick to roll up characters for and to play. Everyone seemed happy enough to be willing to use it again, possibly with a few modifications as mentioned above.

As a GM, I felt that the start and end of the game went quite well, though the middle of it dragged a little bit, mostly because I hadn't really figured out what to do with it. I had an idea for how to start things, and I had a vague idea of where things were ending up, but no good idea of how to get from A to Z. Given that it was a one-off game, it was never really meant to be an open sandbox, so was deliberately quite linear in the way it played out (though the PCs were sensible enough in their precautions at various points to sidestep some of the possible complications).

I think D6 is a good enough system that I also wouldn't mind giving it another go at some point, maybe with a setting closer to the present day. I'd definitely need to play with the combat system some more though, to figure out a better balance to add in a bit more danger for the PCs.