Oil for old tilt trim hydraulic pump

[wynjim55]

I have a late 70's/80's trim unit that seems very common amongst Mercury and Mariner engines, same as the one in this post:

http://www.marineengine.com/boat-forum/showthread.php?395793-Trim-Pump-bleed-help-please-help!

One place I contacted looking for oil suggested I use Castrol Outboard Gear Oil (SAE90) but everyone here says to use 30 weight oil. What would be the problem with using the gear oil?

 

[racerone]

Just plain #30 weight motor oil is what you use !!!!!---------It was the oil specified by the factory.------------Gear oil is not needed and is too thick !!!!

 

[guyjg]

What would be the problem with using the gear oil?

Answer: "Gear oil is not needed and is too thick !!!!''. The pump would be trying to pump Vaseline vs. milk; ain't gonna happen w/o killing the pump motor.