The Oil Options – Engine – Gearbox – Differential preferences are hotly debated! It might even be safer to discuss politics than try and have this discussion amongst the MG community.
Engine and Gearbox (Transmission) #
Here we present some specific data and some personal preferences, what you do is entirely up to you. This is not meant to tell anyone they are wrong. Let’s start with an extract from the owners guide. This information seems to be relatively consistent through the years covering Oil Options – Engine – Gearbox – Differential.
Two things from this information – your usual operating climate/temperatures is a consideration. It is likely that most MGBs are in climates identified at the top of the chart on page 1. Also noted is the API specification “SE”. At the time of MGB manufacturing this was probably the newest most current specification.
The combination of these two things leads you towards selecting 20W50 weight oil that’s API level SE. The SE level specification is old and suggested for cars built in 1979 or before, that combination is hard to find. There was a lot of discussion about Zinc levels being reduced in later API specifications/modern oils. Much discussion of the need for ZDDP additives etc. You can search about that if you wish but oils have been adapted over the years, and MGBs certainly were in an era when catalytic converters were not common until the last few years of production.
Castrol now have a Castrol Classic 20W50 oil that is API Specification SJ. This is recommend for cars made in 2001 or older and apparently has the necessary higher levels of zinc for flat tappet engines. This seems like a fairly good fit for the MGB. I could not find specific product sheets for the Classic oil. I contacted Castrol and they replied:
“Castrol GTX Classic 20W-50 contains a typical Zinc level of 1310 ppm and a typical Phosphorus level of 1199 ppm.”
Brad Penn, Valvoline VR1 and other brands have similar options and specifications and high ZDDP levels. Use the link for VR1 to see their detailed specifications.
The key points are the weight and API specification level and with the API spec the levels of ZDDP. A modern oil designed for current vehicles is unlikely to be a good choice for your MG.
Overdrive Gearbox Additional Thoughts #
John Esposito of Quantum Mechanics Ltd. is well known for Gearboxes and Overdrives in the USA. So, based on research he did he concluded that the best oil for the Laycock LH overdrive is SAE 30W Non Detergent oil. Although they are in the process of rebuilding their website the article/study in question is available on the Web Archive Wayback Machine. This archive of the website has the study/article, just scroll down and look for Topic of the month for June 2005 – Overdrive oil recommendation.
My own experience was that SAE 30W Non Detergent worked well, for a while. Then I started to have issues with the overdrive not disengaging when turning it off. This happened to me on two different overdrive gearboxes. I switched back to regular 20W50 oil and the problems went away. So for me, I use 20W50 in both the engine and gearbox as the owners manual suggests.
If you are trying to diagnose a problem with an overdrive gearbox (transmission), this article on MGB Overdrive Diagnostics should be very helpful.
Rear Differential & Steering Rack #
The owner’s manual seems to specify 90W hypoid oil for the most part with some variations by brand. There are also different “GL” specifications for gear oil from API which are shown on this Castrol website. The short version is that you want to find GL4 gear oil. Although GL5 is more common, as the Castrol site notes:
but typically should not be used in synchromesh-type transmissions, as the EP additives can be corrosive to yellow metals found in those transmission types.
Those “yellow metals” would be the brass bearings in the rear differential.
As noted to the side I’ve had luck finding suitable GL4 85/90W gear oil on Amazon.
Other Materials #
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