I suspect, and it proved true with our -10, that the master solenoid health can be an issue. Another -10 owner a bit north of us also had the same issue.
Getting the right solenoid and mounting it the right way up helps with having good contacts and less drop under load. Or so I am told.
Also in winter on cold mornings oil and batteries don't help. We use Phillips 20W-50 and camguard for what it is worth. (Camguard has nothing to do with this other than we use it for hopefully better corrosion protection).
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