New to big trucks with my 1968 peterbilt 359

Well with the nicer weather I've given her a once over and I'm hauling gravel from the local pit 11 miles away. I've got a couple things I'd like to learn a bit more about.
I've Identified my rear axles as rockwells but how to I identify what type/series of rockwell ? Would like to know a bit more about them.
Also I've heard mixed opinions on if you should run the aux box in over drive with a load. if anyone could weigh in on that, that would be great.

 

Unfortunately my tac doesn't work it's on the mile long to do list lol
I do know that in od on the aux I hit 62mph against the governor in 10th, in direct I hit 49 mph. Lately I've just been leaving it in direct to play it safe it's all country back road so taking it slow isn't a huge deal. I need to fix the shifter so I could at least use od on the way over.

 

OK here's another question, I went to pull the numbers from all the filters since I'm actually putting some miles on it now and I can't find the oil filter. From my understanding it should be on the passenger side mid way on the motor but it's not there and there's no provisions for it. Now I do have a luberfiner but it was my understanding those where only supplemental.

 

Can you tell me more about the "in the pan" oil filter ? On the back of the oil pump is my power steering pump no filters there either, would have noticed that. I get 62mph top speed in 10th with the aux in od, checked it by GPS. The 10 speed goes plenty deep in direct working around the property or in the pit. Appreciate the help normally I'm pretty good at researching things but there doesn't seem to be much out there on these and it doesn't help they where all made differently.

 

It is a direct 10.