THE 6 GUIDELINES ARE:

Never use a used seal in another project as it may develop defects such as cracks or bubbles in the seal lip previously used.
Because of the higher temperature resistance of FKM, this material is also chosen for applications where higher speeds play a role, which raise the temperature at the sealing lip considerably. Usually, using FKM will result in a longer life than using NBR. This compensates the higher price of FKM compared to NBR, as an FKM does not have to be replaced as frequently. The low temperature resistance of standard FKM is limited to -15 ˚C.
Car Engine Oil Seal: Importance and Impact





Because synthetic motor oil can cost two to four times more than regular oil, talk to your technician about whether it’s the right oil for your car. If you live in a climate with super cold winters or very hot summers, or use your vehicle for towing or hauling, synthetic oil may be the best type of oil for your vehicle. Older engines could also benefit from synthetic oil, as it can help prevent harmful sludge build-up that some older engines seem to be prone to.
Place the ruler or straight-edge on edge diagonally across the head and block and look for gaps showing light anywhere between the ruler and block.