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First you want to inspect the inner and outer part of the wheels and make sure you are not leaking any oil. It will be apparent if they are leaking.
On the outside of the wheel should be a plastic cover that holds the oil. On this cover should be a plug you can carefully remove to access the oil. If you just want to change the oil out you can suck it out with a syringe and add new oil. The other way is to remove that plastic cover and all the oil will come out. The problem doing it this way is you will need a new gasket for the cover. IF you decide to do it this way you can get a piece of cardboard folded to catch the oil so it will not go in the wheel. Then you want to raise the wheel spin the tire and re-fill until it is on the full mark. IF you just suck the oil out and re-add fluid drive the unit then re-check the levels... IF you have any other questions or concerns please feel free to contact me..
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These take GL-5 fluid, it does come in a synthetic and I would think the factory did not use the synthetic.
IF this is a stemco cap and you look real close in small lettering will be an add and full mark printed on the cap, it is very important you do not over fill them because you can blow the seals out.
IF you are not having any vibration issues and the fluid smells good not burnt I would personally just top the fluid off and you should be good.