Oil question,?

Lmfao typical big chain filter spinner shop.

I've switched between synthetic and conventional before in several of my own vehicles with no problems. They're just trying to upsell something IMO.

 

Synthetic is like made in a lab so all the molecules are the same size. More expensive up front but longer lasting. Semi synthetic is a mix of synthetic and regular.

Freightliner recommends an oil change every 10-15k with regular oil, or every 50k with semi synthetic.

 

I normally run the delo semi synthetic because that’s what Cummins recommends. If not delo then you can’t go wrong with rotella of course.

 

Cummins recommends valvoline

 

That not being able to switch myth is well over 30 years old. I didn't think it was still being spread.
What is more important then if it's synthetic or not is the temperature range the oil grade is recommended for.
You will hear a lot of people say manufacturer X recommends 10w30 and this is not true. It may come out of the factory with 10w30 but if you read the manual it will tell you to run a oil grade rated for your typical operating ambient temperature range.
10w30 range. -25c(-13f) to 30c(86f)
15w40 range. -15c(5f) to 40c(104f)
0w30 range. -40c(-40f) to 30c(86f)
5w40 syn. Range. -35c(-31f) to 40c(104f)
Synthetic oil is more stable at the extreme ends of the range because of this sometimes syn oils are give a slightly bigger range usually around 5c.

 

Landstar doesn't have him do anything except send in some forms. They have a maintenance schedule form with two options filled in and one blank. For oil change intervals there's options for 10k, 25k, and blank. He can just write 50k in the blank slot and then checkmark that box.

The manufacturer recommended safe point varies by make and model. The current recommendation for your make and model can likely be found on the manufacturer website.