I just use a good premium motor oil and a factory filter. 'High-Mileage' is a common marketing ploy that sure sounds pretty. Synthetic motor oil blends may offer some advantages in heavy-duty/high-temperature applications.
Are you having valve seal issues? What engine?
I plan to run Shell Rotella T in my rebuilt 360-turned-408 stroker, because of the hydraulic flat tappet cam and no-roller-anything. Modern conventional oils lack the heavy metals that helped lubricate those high-pressure bearing surfaces, and I wiped a cam lobe on the old engine in the car because of that.
If an engine is all-roller, I don't expect it to be as big of a problem.
Start with a quality motor oil. HDEO is a good oil. I wouldn't go higher viscosity than 10W-40, I see some people go 20W-50 to slow oil consumption instead of servicing the engine.
Do a dry/wet compression test to make sure that the rings are ok.
Valve seals are fairly easy to do on a 318. Hand tools, a simple valve spring compressor and compressed air to hold the valves up during the operation one cylinder at a time. You may find hardened rubber exhaust valve seals from the heat.