Ed,
No problem with running "dino" oil in the 5.3s. Only the LS2 SSRs are supposed to use synthetic only. Frankly, I'd scoff at paying $70 for the synthetic! I didn't even pay that much for a case of AMSOIL (possibly the best oil out there - period), but then again, I do my own oil changes (you should see my driveway! :lol )
As far as drum brakes on the HHR, it's all about the $. Drum brakes are cheap to make and maintain compared to discs. When you have to factor in the parking/emergency brake hardware, it's easier to simply rig a cable to the drum mechanism than add a complete second set of brake hardware on discs. Also, GM has had tremendous problems with rear discs for some reason, such as on the first gen W-body and on the current full size trucks (GMT-800).