I have a 1998 hp500 that was recently refreshed from top to bottom I was using mobile one 15-50 last year but switched to amsoil this last oil change. The techs at amsoil recommended marine 4 stroke 10-40 I'm alittle leary because I noticed that the amsoil seams to be very thin. Do you think there will be any issues or is there a better amsoil that I should be useing.

what was the reason for the change ? and if you gave the oil guys the correct information about the clearences , loads and temps, then you would have to think that they know more about their product than anyone else...

was there some issue you are trying solve by changing brands ?

I only changed because I have heard good things about the amsoil product. The tech guys never asked about clearance ,load or temps should I have given that info before switching?

the short answer to that is " yes" . when i was dealing with mobil , they wanted to know everything there was to know about the motors before making a recommendation on viscosity and type. they even went into which valve springs we were running ( on the 24 hr motors) because the harmonics would shed some oils quicker than others...
but thats the difference bewteen dealing with engineers and dealing with salesmen.

in my experience and in my best professional opinion, there are no " off the shelf" oils better than the mobil 1 pure synthetics. if you had success with their 15 50 then that implies that that oild was the correct viscosity for the clearences and loads you have. whether by accident or design, you got that correct. unless you have some real non voodoo, genuine, scientific or engineering based, non folklore or sale bullsht reason for changing what you KNOW works, why would you ever do it ?

I use the Amsoil 15/40 marine/diesel oil. After I switched to a roller cam I tried the 10/40 marine, I like the 15/40 because of the 12 TBN also it seems like the oil pressre was a little better after hard runs. Some of the guys here are using the Dominator oil.