#4 drives are rebuilt last year, no issues last year, magnets look good, Only thing I did was replace the oil this year and now the oil in overflow bottle gets aerated after a run.
The other drive no issues.
I pumped oil from the bottom till it came out the top ...maybe some air bubbles?

Maybe your drives are overheating & " releasing/bubbling" from looking at your avatar? She seems to increase my operating temp...

Just a few bubbles or is it turning to foam? I've seen this before where it just puts a bit of bubbles in the oil and after a few outings it subsides. If you are getting a pretty good froth in the bottle that would be a bit concerning.

I agree , its frothy, last time it did that the drive took a dump shortly.
Even after a short 15 minute ride that drive had some signs of heat .

Same oil or different oil ?

If different, maybe the oil you are using now doesn't have the correct , or correct amount, of anitfoaming agents in it. Basically, not made for your app.

Other than that or an air pocket, I 'got nothing' for you. LOL.

Thanx SB ... the other drive is just fine.
I`ll keep an eye on it

agree,Other than that or an air pocket, I 'got nothing' for you.thanks

Make sure your gear lube bottle has the O-ring underneath the cap. Also it could the cap and o-ring. Try swapping them to see if problem stays or follows.

Also some extra info for anyone. Many times we see this issue as the gear lube rises in the bottle and aerates is from when the drive has been worked on internally. The rolling torque specs were set up too tight.

I have the expansion bottles in the picture. I did not realize it but the caps on the drives were air IN only. Meaning there was a baffle to allow air in but not out.. I removed that baffle so they could 'breathe'. Thats the only change since last year when all was fine.
How does the cap and O ring affect the oil?

I believe you want the one way valve in there with the o-ring sealing on the cap. This keeps the drive slightly pressurized when warm keeping the foaming down, and letting air into the system as the oil cools and contracts. Of course you know that the oil will foam if there is any water in it at all. This is going to sound silly; I trim the drive all the way in, jack up the front of the boat and pump oil in the bottom of the drive, I get about another 1/2 quart of oil into the drive. You can see the bubbles in the overflow bottle as the oil is pumped. This is on a wet #6 that holds 18 1/2 quarts. No foaming since I started doing this.