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I started using Rotilla T4 in my 1947 Clipper's 356 cu inch engine. A well know Packard specialist informed me he was using in one of his Packards and it helped quiet down the engine. As my car has some noisy lifters I started using it as well. It did have a significant effect in quieting down the engine.
Around town, the engine has always consistently shown about 40 lbs of oil pressure .
Recently, I had some work done on the car which included having the OD repaired. I drove home, about 1 1/2 hour drive, and was able to easily travel at 65 MPH with the OD engaged. As I got close to home, I noticed the oil pressure had dropped to 20 lbs...I do not know when this started during the drive as I was watching traffic.
A few days later, I drove the car around town and (45-50 mph) and the oil pressure was at the usual 40 lbs.
When I start the car, the oil pressure briefly spikes to 50 which is caused, I think, by the effort of filling the oil filter canister.
My question is would the OD lower the engine RPM so much that the oil pressure would drop to 20 lbs? Or does the 15w-40 oil have some effect here.
As the oil heats up, it becomes 40w...thicker!! That should increase oil pressure, if at all.

that's not what 15-40 means. At the higher temperature it has the viscosity of a 40 at that higher temperature, not a viscosity equal to 40 at low twmperature.

I did not realize that but it makes perfect sense. Thanks!!

i use T4 or mobil delvac in all my cars and trucks even my new car,and lawn mowers,
i buy which is cheaper at walmart at the time, u might of been
lugging the motor down causing pressure to drop, and it will be higher presure when cold,because its thicker, get thin as it gets hot, the oil filter had nothing due with it,
i have a freightliner truck, and i always figured if that oil was good enough for it,it wa good enough for any motor i have
u will save alot on engine wear, i been using those oils in everything for 40 years

Thanks!

When you change oil service, you change not just viscosity profile, but additive package. T-4 is made for DIESEL applications, which have much more pressure, soot, and combustion products to contend with. Those additives can loosen some of the accumulated engine sludge from gasoline.
Personally, I do not like to change oil if the engine has not been rebuilt where you're starting with clean surfaces and passages, no sludge. And rarely would I just take someone's word for adding something without investigating. Can not go by sound alone, either.
These motors don't have the full pressure filter lubrication we're used to today, mostly partial drip filters, if at all. eg; My '49 has no oil filter.
Also, you're only seeing the oil pressure gauge reading, not the actual oil pressure, per se. If it's a mechanical gauge maybe some sludge got in the tube. If sludge gets in the pump intake, it could drop the pressure momentarily too, til it clears out.
If it's electric gauge, maybe the pressure was fine and voltage* dropped and/or the regulator was stuck. Contacts systems are a little bit more unreliable than electronic we're used to today. Plus they're that much older now than new.
My recommendation; do an oil change, ASAP, and maybe several more, if you keep using this oil, til it clears up. I think the manual calls for a periodic pan drop and scrape on some of these with no filter. But maybe not necessary, but can't know the extent without checking.
* - won't take much on a 6V system.

Robert, I did a pan off service when I had the Clipper. Here are photos which you may find helpful. Good to hear the Packard is getting such good care. John

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John,
Thank God for your pics!! You saved me a lot of work.
Weather permitting, I will take the car out on the beltway early Sunday morning for a high speed run. You would not believe the traffic! Rush hour now starts at 2:30!

I got my house to market too late this year! Off soon, then back on in the spring. I cannot wait to move....to many people up here! And Amazon in coming! Madness!!

Great advice!

Robert, I recall that the temp gauge in the inst panel read on the low side (or was it on the high side?), and that an auxillary gauge had been installed for a more accurate reading. Perhaps the oil pressure gauge is doing the same thing? We are well settled after moving from Austin to Santa Fe a year ago. Enjoying the smaller scale of Santa Fe and the less than crazy traffic. Good luck on the sale and move. John