Motor oil and I like to add a little Mavarls mystery oil in there just to plump up the seals a little.

Hmm, maybe that will fix the leak I'm getting from either the pump or rams since it's only about 1" per week. I'll dump some in this weekend!

Also noted the manual says a Oildyne pump use Merc. Trim/Steering fluid or Dexron 2 which is no longer available. Not sure if the Merc. Is synthetic or not but the D2 is not and substituting a higher Dexron level (which is synthetic) may cause issues (by definition).

To tell the truth I have 4 trim pumps in 3 boats and 2 use motor oil and 2 use trans fluid. Just the way they came.

I started using at least some Marvels in all my trim pumps a few years ago after it fixed a trim ram leak. I don't believe in snake oils but I remember reading online decades ago that farmers kept gallons of transmission stop leak on hand so they could soak their fan belts, seals and such in it to restore them, back in the day when a fan belt may take 2 weeks to get. So today I don't keep gallons but I always have a bottle on hand in case I have a seal that needs restoring and it works. Also when ever I rebuild a motor a Qt goes in. New trans, it goes in. I'm sold on Marvels. It's not going to fix a broken rod but a seal that may have hardened up on a trim ram it just may.

Sold on Marvel as well after adding it to the crank case on one of my 7.4 LX Mpi's that had lifter tick. I knew the motor sat for ~1.5 years which is a telltail cause of tick. I also knew being all OE and Only 227 hrs. There was no way the lifters were worn. Used marvel per the directions, Ran the motor for 2.5 hrs., Drained and no more tick.

Also use Prolong oil treatment after a friend witnessed a 6 sec. drag car dump all its oil on the track and finish the run with little to no oil pressure. Upon tearing down the engine, zero damage from crank to top end! May be a fluke but at $20 ea. Per change, and typically on sale buy one get one, I don't mind the extra $ for peace of mind.