Pennzoil does make a true synthetic oil, where most other synthetic oils are not actually real synthetic. Most full synthetics are made from crude oil. Pennzoil uses natural gas which is a cleaner base. Pennzoil ultra platinum is supposed to be a real good oil. I just like Mobil 1 because I have history with it, and haven't had issues with it.
Research I have found Mobil 1 may not swell. It uses a thicker base oil to help protect against leaks. " formulated with a higher base oil viscosity to help reduce leaks and provide better high temperature protection" (Mobil 1™ Extended Performance High Mileage 5W-30
"The function of seal conditioners in ALL of our oils is to keep your soft seals flexible and pliable. This allows them to conform to little surface irregularities and machining imperfections, to keep the oil on the inside. My High Mileage oils contain a slightly higher conditioner content, intended to help if we have a problem seal." (https://bobistheoilguy.com/forums/threads/seal-conditioners-mobil-1.352878/
"I work in the rubber elastomer sealing business, and when I hear seal swelling, I think of a seal that is being compromised, or degraded, and will be failing shortly. In the short run, it may be stopping a leak, as it is filling in a greater area. A torn seal or a worn sealing surface will be fixed temporarily. But the seal will need replacing eventually, and the sealing surface checked for wear. I think it would be unusual for a seal to be exposed to anything in or on an engine to cause it to swell, or degrade, but of course anything can happen. As far as seal conditioning, as has been mentioned with doing an Auto-RX treatment, there is no swelling. Instead, the sludge creating a false seal is cleaned/removed, exposing the actual seal to to the conditioning elements of the oil. It is no longer "drying out", and hardening, from lack of oil. Seals need lubrication as much as bearings. The seal then is able to reconform by returning back to its original softness/hardness. This is a durometer measurement. Now, if the false sludge seal is covering a tear or a damaged or brittle seal, obviously there would be a leak problem. One of the side benefits of some of the products mentioned on this board, in their maintenance doses, is to allow a seal to conform or stay conformed, in additional to keeping things clean." (Seal Swelling vs. Seal Conditioning
Looks like they don't actually swell the seals. They recondition like at-205, but I wonder if it is the same or different. Appears to have the same effect. With the video below it shows Mobil 1 at least doesn't swell the seals at all. So I would assume these conditioners are not swelling. Although there are some chemicals that do indeed cause swelling, I don't think the engine oils are putting in these chemicals. I have heard brake fluid can swell rubber and has been used to seal engines, but ultimately cause damage if done incorrectly (Left in too long, or put in too much).