Engine oil choice 1984 3.8l

[Ristin]

I'm the 3rd owner of a 1984 Seaswirl Sierra with a 3.8l OMC stringer 800. The boat was very well taken care of and came with maintenance records from date of purchase to current. I even have the original owners purchase documents and all owners manuals for the boat, power train and trailer.
The manual recommends straight SAE 30 engine oil and is very insistent. There is an entire page in the engine manual devoted to oil weight choice. The 1984 manual acknowledges that multi-weight formulations have improved but SAE 30 is still the best choice.
I acknowledge that oil formulations have improved significantly since 1984, but I still wonder the reasoning behind the SAE 30 requirement.
SAE 30 oil is getting more difficult to find on the shelf. Locally I can only find it at Tractor Supply and North 40.
I continue to run SAE 30 but wonder if multi-grade and synthetic blends will be harmful to the engine.
Thoughts?

 

[Scott Danforth]

SAE 30, 15W-40, 20W-50 will all work

Your tilt system and power steering system also use SAE 30. You can drain those systems and swap to ATF.

However most likely your lawnmower and pressure washer also need SAE 30. I buy the big 2.5 gallon jugs at TS

I would also look at swapping out the batwing manifolds if they haven't been converted to the 2-piece manifold/elbow

 

[Scott06]

The only thing I might add is you have a flat tappet cam, some modern oils do not have enough zinc or ZDDP for flat tappet cams as most engines are roller lifters now.
Pretty sure Rotella T4 (conventional not sythetic) still does have enough zinc for flat tappets. Not only do a lot of guys here use it but also popular in the 60's muscle car crowd. A lot of guys on the GTO forum I'm on have used if for years no issue

It also happens to be very reasonably priced. If it were mine I would use Rotella T4 15W40 c0nventional and call it a day

 

[Ristin]

Thanks (belatedly) for the replies
I'll try the Rotella T-4 15w-40. I already run that in my 1993 7.3L idi turbo diesel pickup.

"However most likely your lawnmower and pressure washer also need SAE 30. I buy the big 2.5 gallon jugs at TS" This statement made me think for a minute. Looked up all the air cooled power toys/tools and found only my push lawn mower uses SAE 30. Everything else recommends 10w-30.

"I would also look at swapping out the batwing manifolds if they haven't been converted to the 2-piece manifold/elbow" Great. Something else to worry about. Thanks for that. hahaha
Attached a pic of the manifold:

 

[Scott06]

Thanks (belatedly) for the replies
I'll try the Rotella T-4 15w-40. I already run that in my 1993 7.3L idi turbo diesel pickup.

"However most likely your lawnmower and pressure washer also need SAE 30. I buy the big 2.5 gallon jugs at TS" This statement made me think for a minute. Looked up all the air cooled power toys/tools and found only my push lawn mower uses SAE 30. Everything else recommends 10w-30.

"I would also look at swapping out the batwing manifolds if they haven't been converted to the 2-piece manifold/elbow" Great. Something else to worry about. Thanks for that. hahaha
Attached a pic of the manifold:
View attachment 395944

Those are not the batwing manifolds that had casting issues and can leak water into engine.

If they are not corroded or cracked leave them alone. Most freshwater ones last the lifetime of the boat if they don't get freeze damaged