Did you test the batt. under load? Turn on headlights, radio,etc
Computer needs at least 10V to function.
A weak batt. will fall under ten V while starting.
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One engineer stated the the advantage of thinner oil was easier starting in winter. I do not change weight, just a minute or so of idling, then drive moderately untill normal temp is attained.
Beware "miracle additives" to modern engines the synthetic oils are a very complex brew, some items...