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Engine rpm's are more of a concern than actual speed. Regardless you should not have to adjust driving habits to preserve the life of engine seals (assuming the driver is not an all out hot rodder).
The actual causes-
1. internal engine imperfections damaging the seal
2. time overcoming the usefull life of the seal
3. adding engine oil additives (stop leaks especially) or accidentally adding transmission fluid to the engine.
Number 1 often goes un-diagnosed and causes repeat failure.
1 and 2 are best prevented by proper maintenance, which includes regular oil and filter changes. Quality problems with engine parts and seals is beyond your control.
This site is underutilized for asking advice on the purchase of a car. When you shop around and find a couple cars you really like, stop by and ask us what we think. We'll let you know what our experiences lead us to believe you can expect for quality...