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In 30k do they have the gear symbol on the shoulder or is that a 40k thing

The gear icon is the symbol of the Adeptus Mechanicus and is usually used in the 41st Milennium to denote Techmarines who have been trained directly by the Adeptus Mechanicus. As I understand the lore vehicle crew members don't always have the gear icon, those that do are actually AdMech-trained Techmarines themselves who happen to be attached to the Armoury rather than wandering around the field with a servo-arm fixing things.

Given the presence of the gear icon sculpted onto Forge World's Mk.III and Mk.IV Techmarine models I'd expect that to be a tradition dating back to the Legions, so your vehicle crew may or may not have gear icons, it indicates their Mechanicum training rather than any immediate affiliation with the motor pool.

As a point of fluff: The Legions were much more capable in manufacturing and maintaining their own equipment. They were not wholly dependent on Mars or the other Forge Worlds. They were much more like today's modern military. New stuff gets purchased, a couple guys go to the manufacturer and learn everything they can about the stuff, then they go back to the unit and train other guys on the new stuff. As the stuff breaks, they fix it, not the manufacturer (unless it's something really, really complicated, at which point they just might buy a new one). Part of that fixing might involve making a part, and depending on the part, the unit's machine shop should be able to produce it.

For example, the Rhino is based on an STC design, and only the Forge Worlds have the STC machines. So the Forge Worlds make a bunch of Rhinos. A Techmarine travels to the Forge World where they train him on how to use the Rhino, how to fix the Rhino, etc. etc. To show that he has been "Officially Trained", they give him a badge- the cog symbol. (Happens all the time in the US Military every time new equipment rolls out). He goes back to the Legion home world, and trains the other Techmarines on the Rhino. Rhino's arrive, they go out and make war. As the Rhino's break down, the Techmarines fix them as much as they can. Sometimes it'll cost too much to fix a Rhino, and other times, only the Forge World can replace the complex cognitive brain of the computer. That's how it works.

Occasionally, you get a really, really smart Techmarine that figures out how to add a lascannon to a Predator turret, and lascannon sponsons creating a Predator Annihilator- not an STC design, but born out of necessity on the battlefield. BTW, it was a Space Wolf Techpriest that made the first Predator Annihilator out of some Dreadnought parts and a Devastator squad's Lascannons. Fun little story.

TLDR; Yes you can add the Cog symbol to your 30K Space Marines and still be fluffy.