I would put a mechanical gauge on it and find out what the oil pressure really is at idle and at crusing speeds.
From that you can make a determination if there is an oil delivery issue or not.
Typically slant six oil pumps are robust, but the oil pressure bypasss valve should be serviced every 100,000 miles or so.
The squeaking could be an oiling issue, like maybe there is no oil getting to the rockers,,,anything is possible, so put a mechanical oil pressure gauge on it and respond with the oil pressures.
And mixing 15w40 and 10w30 is not the cause of the issue.
