I am going to Rausch Creek this weekend and wanted to check my diff fluid since they continuously leak a bit like they all seem to. I know that you should be able to feel the oil level with your finger in the hole, but was wondering what other tricks there might be. What type of oil is best for off roading (Royal Purple?). Is there any issue with using the oil with additive on a non limited slip diff? Etc.
For heavy duty use, synthetic is the way to go. Try some Quaker State 75W-140 FULL (not blended) synthetic. 75W-140 is recommended in my Owner Manual for trailer towing.
No need to be "additive happy" unless they are specified in your Owner Manual. Most additives are a hyped up waste of money. The fluids used now days, that meet specifications, have an additive package in them already such as antiscuff additive for example.
From Wikipedia on gear oils:
Gear oils are classified by the American Petroleum Institute using GL ratings. For example, most modern gearboxes require a GL4 oil, and separate differentials (where fitted) require a GL5 oil. It is important that purchasers check the oil against the vehicle manufacturer's specification to ensure it does not contain any aggressive chemicals that may attack gear components, such as the phosphor bronze used in many Japanese boxes.
API viscosity ratings for gear oils are not directly comparable with those for motor oil, and they are thinner than the figures suggest. For example, many modern gearboxes use a 75W90 gear oil, which is actually of equivalent viscosity to a 10W40 motor oil. Multigrade gear oils are becoming more common; while gear oil does not reach the temperatures of motor oil, it does warm up appreciably as the car is driven, due mostly to shear friction (with a small amount of heat conduction through the bellhousing from the engine block).
Fully synthetic gear oils are also used in many vehicles, and have a greater resistance to shear breakdown than mineral oils. They can improve the shifting performance of "difficult" gearboxes, where the excessive slipperiness of some mineral oils can impede synchromesh action.
API classification subdivides all transmission oils into 6 classes:
API GL-1, oils for light conditions. They consist of base oils without additives. Sometimes they contain small amounts of antioxidizing additives, corrosion inhibitors, depresants and antifoam additives. API GL-1 oils are designed for spiral-bevel, worm gears and manual transmissions without synchronizers in trucks and farming machines.
API GL-2, oils for moderate conditions. They contain antiwear additives and are designed for worm gears. Recommended for proper lubrication of tractor and farming machine transmissions.
API GL-3, oils for moderate conditions. Contain up to 2.7% antiwear additives. Designed for lubricating bevel and other gears of truck transmissions. Not recommended for hypoid gears.
API GL-4, oils for various conditions - light to heavy. They contain up to 4.0% effective antiscuffing additives. Designed for bevel and hypoid gears which have small displacement of axes, the gearboxes of trucks, and axle units. Recommended for non-synchronized gearboxes of US trucks, tractors and buses and for main and other gears of all vehicles. These oils are basic for synchronized gearboxes, especially in Europe.
API GL-5, oils for severe conditions. They contain up to 6.5% effective antiscuffing additives. The general application of oils in this class are for hypoid gears having significant displacement of axes. They are recommended as universal oils to all other units of mechanical transmission (except gearboxes). Oils in this class, which have special approval of vehicle manufacturers, can be used in synchronized manual gearboxes only. API GL-5 oils can be used in limited slip differentials if they correspond to the requirements of specification MIL-L-2105D or ZF TE-ML-05. In this case the designation of class will be another, for example API GL-5+ or API GL-5 LS.
API GL-6, oils for very heavy conditions (high speeds of sliding and significant shock loadings). They contain up to 10% high performance antiscuffing additives. They are designed for hypoid gears with significant displacement of axes. Class API GL-6 is not applied any more as it is considered that class API GL-5 well enough meets the most severe requirements.
2003 Jeep TJ Sport - 4.0L - 251,430 kms
2002 and 2007 Grand Caravans - Sport & SXT