If the car has been properly maintained (regular oil changes) you should be perfectly safe switching to a synthetic at 70K miles. It's engines that have been abused and/or have high mileage (100-120K+) that you have to worry about. Even then, it's only a CHANCE they might leak. Synthetic oils have come along way in preventing leaks in the last 10+ years.
Leaks, however rare, usually occur around the pan, main seals, etc ...
**NEVER** use an engine flush. These things are almost guaranteed engine-killers. Many places don't even sell these anymore.
I don't recommend ANY of the synthetic blends on the market. They usually have only 10-15% synthetic in them ... at 50% of the price of synthetic.
Your oil consumption should go down noticeably if you switch from dead-dino juice to synthetic oil.
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Bror Jace