The first thing is verify with a mechanical gauge. It is normal for oil pressure to drop at idle after warmup. If this is a "new" condition, further investigation will be required.
Now, understanding why oil pressure may be low is another thing altogether. Here are some causes:
1. Plugged oil filter - yes it happens. Replace oil and oil filter (see #3).
2. Pressure regulator in the oil pump sticking open at times. With this one you will see the needle "pulse" at idle as the gears in the pump work against each other. You can try something like Rislone or Marvel Mystery oil top help dissolve any varnish that might be causing the problem. Other than that, a new oil pump may be required. Judging by your first post, if it does this on a mechanical gauge, I'd say this might be your problem. The pressure regulator is sluggish and hanging open momentarily. Give the oil additive a good chance to work before dropping the pan.
3. Clearance between bearing surfaces. An engine lubricating system is based on pressure and volume. The combination of the fit between the bearings (Mains, Rods, Camshaft) and the associated shafts can have an effect on oil pressure should the pump volume be exceeded due to excessive "squeeze-out" between them. To determine if the bearings are wearing have an oil sample taken and an oil analysis done. If the bearings are going it will show up in the lab results. While this sounds scary, it's seldom the case without other obvious indicators (loud banging metal sounds, parts that should be inside hanging outside, etc.).
I did have an engine once that had 20psi running down the road and 8 psi at idle on a new rebuild. Engine lasted over 200,000 due to the increased lubrication and cooling effect by greater oil flow over the bearings.
By the way, been working on engines since 9 years old. Career marine mechanic for over 25 years.
Hope this helps.
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