Can I use synthetic in a brand new engine?

I just rebuilt an engine for a 91 ford ranger 2.3L.
My manual says to use 5w-20 conventional but since there have been some advancements in the synthetic oils since the 70's, do I need to worry about seal swell or shrink? My instinct is to just use conventional to play it safe, but I'd really like to use synthetic.
I'm reading conflicted information…

I just rebuilt an engine for a 91 ford ranger 2.3L.
My manual says to use 5w-20 conventional but since there have been some advancements in the synthetic oils since the 70's, do I need to worry about seal swell or shrink? My instinct is to just use conventional to play it safe, but I'd really like to use synthetic.
I'm reading conflicted information… Synthetics don't swell seals, unless it's labeled as a high mileage oil. In high mileage oils, they put additives that swells seals so the engine doesn't leak.

If you want to extend the oil change interval use synthtetic. If you are going to follow the normal change interval just stick with regular. This conventional oil versus synthetic debate can only happen in the US. When I took my cousins car for an oil change in Eastern Europe, I asked the dealer to use synthetic, the dealer laughed and said they use nothing else but synthetic. In fact they have not used mineral based oil for over 6 years. All European cars come factory filled with synthetic. Check the owners manual and use the recommended oil. It is not differential calculus (aka rocket science). The FIRST advance in synthetic oil since the 1970s is the INVENTION of synthetic oil. The manual doesn't say you should use conventional oil. It says you don't HAVE to use synthetic oil. I would break it in with conventional oil and run it Maybe change it after 500/1000 miles and put conventional oil in it thru break in period of up to 4000 miles and then change to synthetic if you want. You want it to break in before you put the slicker oil in it.

Some say do and some say just put synthetic in from the go. So you read and decide. Just stick with the recommended type of oil. Using synthetic oil in a new motor is not a good idea for the first 1000 miles. It doesn't allow the piston rings to seat properly in the cylinders. You can switch to it after the first 1000 miles with no problems. I've never seen seals swell or shrink form using regular motor oil. Recall the era when it was recommended, which I followed then because synthetics were not an option, to change oil in a car that was in "break-in" stage at 600 miles. New cars will likely use a blend of some kind wen they are sold but the first oil change should be quality synthetic for all kinds of reasons.

This conventional oil versus synthetic debate can only happen in the US. When I took my cousins car for an oil change in Eastern Europe, I asked the dealer to use synthetic, the dealer laughed and said they use nothing else but synthetic. In fact they have not used mineral based oil for over 6 years. All European cars come factory filled with synthetic.

Check the owners manual and use the recommended oil. It is not differential calculus (aka rocket science).

The FIRST advance in synthetic oil since the 1970s is the INVENTION of synthetic oil. The manual doesn't say you should use conventional oil. It says you don't HAVE to use synthetic oil.

I would break it in with conventional oil and run it Maybe change it after 500/1000 miles and put conventional oil in it thru break in period of up to 4000 miles and then change to synthetic if you want. You want it to break in before you put the slicker oil in it.

Some say do and some say just put synthetic in from the go. So you read and decide.

Just stick with the recommended type of oil.

Using synthetic oil in a new motor is not a good idea for the first 1000 miles. It doesn't allow the piston rings to seat properly in the cylinders. You can switch to it after the first 1000 miles with no problems. I've never seen seals swell or shrink form using regular motor oil.

Recall the era when it was recommended, which I followed then because synthetics were not an option, to change oil in a car that was in "break-in" stage at 600 miles. New cars will likely use a blend of some kind wen they are sold but the first oil change should be quality synthetic for all kinds of reasons.