To james.oco200, I will assume that the oil leak repair in question was to replace the bedplate seal, since you mention the cab was lifted and the engine removed for this repair (it is also the only oil leak repair that comes to my mind, requiring disassembly to that extent). For what it's worth, to replace EGR/oil coolers AND head gaskets under WARRANTY time, pays 25 flate rate labour hours. Again, that is WARRANTY time, not retail. Typically, when quoting a RETAIL repair (meaning a the vehicle's warranty has expired), most places will quote labour hours that are between 30 to 40 % more than what the WARRANTY time pays.
Off topic, to replace a bedplate seal requires engine removal, and mounting it on a stand to be turned upside down. This process also involves removing the front and rear covers of the engine, in order to enable removal of the bedplate itself. It DOES NOT require removal of cylinder heads.
Since you REQUESTED the servicing tech to install head studs, this repair now necessitated removal of the cylinder heads (along with the injectors and all associated valvetrain parts), and then the cleaning of gasket surfaces necessary to prepare for re-installation, you've now added to the repair he was doing. 12 hours of labour is about HALF of the 25 labour hours he would've been paid if he were doing just the head gaskets and coolers under the TSB. That's right, HALF of the WARRANTY time, since it was already apart about half way. That said, I think it was pretty fair.
To the guys comments that "any mechanic" should be able to tear it apart in 2 hours and re-assemble for just a little more than that, I can only ask what kind of a "hack" would you rather have wrenching on your ride? Even if it didn't take the actual time charged, this is what flat rate is all about. How long it actually takes is dependent soley on the servicing tech. If a job PAYS 4 hours, but takes actually 1 hour or 7 hours, that is what the tech gets paid (and what the customer is charged) 4 hours. For those who feel it unfair that they paid labour hour charges greater than the actual time the job took, look at it this way. You're not paying for the actual time the job took to carry out. You're paying for the time it took for the tech to acquire the experience/knowledge to be able to carry out the job in less time than the labour time you paid.