"When I press the starter it engages, then typically cuts out making a high pitched "Whirring" noise."
The noise is the starter spinning after the bendix prematurely disengages.
You can pull the starter, check the shaft & clean it up (no lube).
If that doesn't work, pull the starter (see tip # 36) & replace the bendix.
" Seller just changed oil and used Shell Rotella 15W40. Any problem with that? (fwiw, I'm in central PA)."
The oil is fine.