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OK, picked up a couple of gallons of synthetic 50w manual trans oil. My question is do I just drain out the old 90w and refill with the synthetic or must I somehow flush out the remaining 90w residue?

I'd say if you are really hoping to gain the advantages of better oils, then just do a second fluid change a short while later. It is not like they won't mix, like oil and water. You just want to dilute any remaining conventional oils down to a negligible level

you should be fine just running it as is. not going to hurt anything as long as the oil you got is compliant with brass syncros. You can wipe out what residue you can get at and that will be plenty. Do another oil change in a year or so and all will be well

I did my fluid swap while I had the top cover off, and I used some break cleaner and rags to clean out the residue in the bottom of the case, but all in all it should be fine with a drain and re-fill.

The truth is you are asking the impossible question. Oil additive packages that are blended in all quality oils are made up of numerous chemicals. If you mix chemicals that are incompatible with each other, honestly there is no telling what kind of a reaction that could happen. You could create a monster in your gear box. I understand you are speaking of changing to synthetic, truth is it could just as easily happen when changing out to a different petroleum oil lubricant as well. The best policy is to clean out every bit of residual oil that you can. Removing the top and side covers for access is advisable. Aerosol brake cleaner works well for washing off residue, then blow it dry with compressed air when it's clean. This also enables you to visually inspect the condition of the internals to see that all looks healthy, and functions properly before you put in new oil.

MSeriesRebuild wrote:The truth is you are asking the impossible question. Oil additive packages that are blended in all quality oils are made up of numerous chemicals. If you mix chemicals that are incompatible with each other, honestly there is no telling what kind of a reaction that could happen. You could create a monster in your gear box. I understand you are speaking of changing to synthetic, truth is it could just as easily happen when changing out to a different petroleum oil lubricant as well. The best policy is to clean out every bit of residual oil that you can. Removing the top and side covers for access is advisable. Aerosol brake cleaner works well for washing off residue, then blow it dry with compressed air when it's clean. This also enables you to visually inspect the condition of the internals to see that all looks healthy, and functions properly before you put in new oil.

Well there you have it from someone who knows a lot more than me. Disregard my post.