No one mentioned roundabouts. They're easy to negotiate. Slow down as you approach a roundabout, and yield to any cars already in the roundabout. If you're going straight out of the roundabout, keep to the left in the roundabout. If you're going to be turning right out of the roundabout, get in to the right lane of the roundabout (if there are two lanes), and keep an eye out for anyone encroaching on your left.
Some of the larger roundabouts have directional markings on the pavement, if there two lanes approaching the roundabout. The A90 roundabout approaching Dundee is one such roundabout. And as you're going to be in Aberdeen, you may encounter the notorious Haudagain Roundabout, which has been voted the worst roundabout in Britain. It's at the bottom of Anderson Drive, where the A90 and A96 meet. Make sure that you're in the correct lane well before you reach it.
On the single track roads with passing places, the "pull out" or passing places can be on either side of the road. It mainly depends on where the road builders could find a spot with no subsidence. Oddly enough, the highway code dictates that a car coming uphill has the right of way. Nothing beats the thrill of having to back uphill to the nearest passing place!
You won't have any trouble finding petrol stations. Petrol is sold by the litre, so what you may think is a low price isn't. Petrol is currently running about $8.00 per U.S. gallon, but most hire cars get really good m.p.g. Skye is the only place you'll be where filling stations are few and far between, so I'd recommend filling up at the filling station in Broadford, next to the Co-op supermarket. It's about the best price you'll pay on Skye, and it's the easiest in and out. On the mainland, the cheapest petrol is usually at the filling stations connected with supermarkets, i.e.: Tesco, Asda, Morrisons. They often have price wars.
As in the U.S., rain doesn't affect driving. The Scottish roads are well maintained, but common sense would tell you to drive slower, especially as you're approaching curves.
As they used to say in the ESSO commercials, "Happy Motoring!"